Defending the Trinity from a Thomistic Perspective
Introduction
The doctrine of the Trinity – that God is one in essence and three in person
– stands at the heart of Christian faith and yet has long been a focal point of
controversy and misunderstanding. From the early fourth-century debates with
Arians who denied the Son’s full divinity, to Islamic critiques that the
Trinity compromises God’s unity, believers have been challenged to explain how
one God can subsist as Father, Son, and Holy Spirit. At first glance the
doctrine may seem paradoxical or even irrational: How can “three” be “one”
without violating the law of non-contradiction? A Thomistic approach, drawing
on the metaphysical insights of St. Thomas Aquinas, offers a rigorous and
coherent framework for addressing these questions. Aquinas’ teachings on divine
essence, relation, and procession allow Christians to articulate the Trinity in
a way that preserves God’s divine simplicity and unity while
accounting for the real distinctions between the Father, Son, and Holy Spirit.
In what follows, we will present and defend the Trinity using Aquinas’
metaphysics, explaining how multiple divine persons exist in one essence
without division or composition. We will engage historical objections –
specifically the Arian claim that the Son is a created being and the Islamic
claim that the Trinity implies polytheism – and show how a Thomistic
understanding answers these concerns. Throughout, the goal is to demonstrate
that while the Trinity is ultimately a mystery of faith, it is
not a contradiction of reason. On the contrary, with the help of sound
philosophy we find the Trinity to be a sublime truth above
reason yet not against it, inviting us to an ever-deeper
contemplation of the one God in three Persons.
Divine Simplicity and Triune Unity
Central to Aquinas’ defense of the Trinity is the doctrine of divine
simplicity, the teaching that God is not composed of parts or diverse
properties. In God, there is no composition of matter and form, substance and
accident, or essence and existence – God is His own essence and
existence, utterly one and indivisible (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)) (Trinity
and Simplicity — The Reformed Classicalist). Any division or composition in
the Godhead would imply dependency or change, contradicting the absolute aseity
and perfection of God. The challenge, then, is how to affirm three
persons in one simple divine essence without introducing division.
Thomas’s solution hinges on a careful distinction between how things are in God
in reality and how we conceive them in our minds. The
classical maxim, affirmed in scholastic theology and by the Council of
Florence, is that in God “everything is one where there is no opposition of
relationship” (Catechism
of the Catholic Church | Catholic Culture). In other words, any feature of
God that does not involve a relational distinction is identical in all three
persons. Father, Son, and Holy Spirit are one being, one nature, one
mind, one will – indeed one God – because they share the same simple essence.
There is no partitioning of the divine substance among three “parts”; each
person is the fullness of the one God. As one theologian explains, “All that is
in God is God,” and the three persons are not composite parts of God but each
is the identical divine substance subsisting in a relational way (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)) (Trinity
and Simplicity — The Reformed Classicalist). Thus, divine simplicity is not
violated by the Trinity: whatever is true of God’s essence (such as eternity,
power, goodness, mind, will) is true of the Father, Son, and Spirit equally and
indivisibly. The persons are really distinct from one another,
but not distinct as separate substances. They differ relationally (as
we will see below), not by having different natures or attributes. In God,
essence and person are one and the same reality, and only the relations of
origin differentiate the persons (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)).
Aquinas emphasizes that the divine essence “is not really distinct from person”
(SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39));
the Father’s Godhead is numerically identical to the Son’s Godhead
and the Spirit’s Godhead. The unity of God therefore
remains absolute: one essence, one “God,” in three
who subsist in that essence. Any appearance of contradiction (“three in one”)
is resolved by understanding that “one” refers to the being or essence
of God, while “three” refers to the persons or hypostases,
distinguished by their relations. The Trinity is one as to what
God is, and three as to who God is – a crucial
distinction that keeps us from both polytheism (which would falsely multiply
the essences and give three gods) and modalism (which would falsely
collapse the persons into mere roles of one person).
Procession and Relation in Aquinas’ Theology
How exactly are the divine persons distinguished without compromising God’s
unity? Aquinas answers: by relations of origin grounded in two
eternal “processions” within God. A procession, in Thomistic terms, is not a
movement through space or a coming-into-being (as it is with created things)
but an internal emanation that remains within the divine nature (Aquinas)
(Aquinas).
Christian revelation speaks of the Son as “begotten” of the Father and of the
Holy Spirit as “proceeding” from the Father (and, in Western theology, from the
Father and the Son). Aquinas seeks to elucidate these truths
philosophically. He identifies two and only two processions in God: one by way
of the intellect and one by way of the will (Aquinas)
(Aquinas).
The first is the generation of the Son. God the Father
eternally knows Himself, and by this perfect act of intellect
He generates the Word – an interior utterance or concept that
fully expresses His essence (Aquinas)
(Aquinas).
In John’s Gospel, this divine Logos (Word) is “with God” and “is God” (John
1:1), indicating both distinction and unity. Aquinas explains that when a mind
understands something, it forms an inner word or idea, which proceeds from the
mind yet remains within it (Aquinas)
(Aquinas).
In an analogous (though immeasurably higher) way, the Father’s act of
self-understanding “proceeds” to a perfect image or Word of Himself –
that Word is the Son. This procession is called generation
because it is like a parent communicating life to offspring, except here the
“nature” communicated is the one divine essence itself. The Father eternally
begets the Son by knowing Himself, and in that single act the Father gives the
fullness of the Godhead to the Son. There is no “time” when the Son did not
exist, for this generation is eternal; nor is the Son a lesser copy, for He
receives the identical divine nature. As the Nicene Creed professes, the Son is
“God from God, Light from Light, true God from true God, begotten, not
made, consubstantial with the Father.” Thomistic metaphysics reinforces
this: because this procession is entirely internal to God, it does not
result in another being or a created effect, but rather in another subsistent
relation within the one God (Aquinas)
(Aquinas).
All that is in the Father – the simple divine essence – is communicated to the
Son. “All that exists in God, is God,” Aquinas writes, so every internal
procession necessarily shares the one divine nature (Aquinas).
The Son, proceeding as the Word, is God, co-equal and co-eternal with
the Father, distinguished only by being Son (the one who is from
another) rather than Father (the one who is from no one).
The second procession in God is the spiration of the Holy Spirit,
an eternal act of the divine will or love. Just as God
perfectly knows Himself, He also perfectly loves Himself.
Aquinas describes how in an intellectual nature (like God’s), besides the
intellectual word there is also a procession of love: “the object loved is in
the lover” through an act of will (Aquinas)
(Aquinas).
The Holy Spirit is precisely this mutual Love or Gift personified – the love
that flows between Father and Son (or from Father through Son) from all
eternity. The Spirit’s procession is often called spiration
(breathing forth) or simply procession. It is analogous to the
way our will, upon knowing something good, produces an inner movement of love
toward that good. In God, the Father and Son together breathe forth the Spirit
as the one Love that they share. Importantly, this second procession is also
entirely internal and does not leave the divine essence; it is the immanent
fruition of God’s self-love. Consequently, the Holy Spirit too receives the one
indivisible essence – He is God, on equal footing with Father and Son.
The three persons are sometimes summarized as lover, beloved,
and love, or as mind, word,
and will in an infinite, absolute degree. These analogies
(going back to St. Augustine) help us grasp how plurality of persons does not
entail separation in substance. The processions of intellect and will in God
terminate in relationships rather than independent beings,
much as a thought or act of love remains within the thinker or lover.
According to Aquinas, the divine processions give rise to real
relations in God: Fatherhood, Sonship, and the relation of Spiration
(between the Father and Son on one side and the Spirit on the other). These
relations of origin are the only basis for distinction in God. The Father is
related to the Son as begetter to begotten; the Son to the Father as begotten
to begetter; the Spirit to the Father and Son as proceeding from them. These
relationships are mutually opposed – for example, paternity is
the opposite of filiation; being the principle of procession is the opposite of
proceeding. Such relational opposition is crucial: it allows
us to say the Father and Son are truly distinct from each other
(because the Father is not the Son (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)),
the one begetting is not the one begotten), even though each is the same God.
Outside of the opposition of relations, everything in God is identical. Aquinas
puts it succinctly: considered in relation to the divine essence, these
relations are not really distinct from the essence (since in a simple
being, what God has God is); but considered in relation to
each other, the relations are really distinct (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)).
The persons just are the subsistent relations in God’s being (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)) (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)).
Thus the Thomistic explanation is that Father, Son, and Holy Spirit are distinguished
by their relations of origin alone – an eternal web of knower, known,
and love – and not by any composition or division of the divine substance. The
persons co-inhere in one another in perfect unity of being (per the
Greek Fathers’ term, perichoresis, and as later Latin theology
phrased, “the Father is wholly in the Son and in the Spirit, and likewise the
Son in the Father and Spirit, etc.” (Catechism
of the Catholic Church | Catholic Culture)). We can say the Father is
God, the Son is God, the Spirit is God – each wholly and
eternally – without saying the Father is the Son or the Spirit,
because “Father” and “Son” designate a relationship, not a separate essence. In
sum, by Aquinas’ account the Trinity involves one essence
(God) and three subsistent relations (the divine persons). The
unity is preserved because essence is one; the plurality is real but resides
entirely in the order of relationship. God is not one thing and three
things in the same respect, but one infinite reality existing as three
relative modes of subsistence. This is how multiple persons exist in one simple
essence without violating simplicity: the persons are not extra
ingredients or properties added onto the essence, but are the one essence
itself, differently oriented by relationship. As the Fourth Lateran Council
taught and Aquinas often repeated: it is the relations that distinguish the
persons, and apart from the relations, everything is one in God (Catechism
of the Catholic Church | Catholic Culture).
Answering Arian Objections: The Son’s Eternal Divinity
One of the earliest major challenges to the Trinity came from Arianism,
which contended that the Son (and by extension the Holy Spirit) are not
co-eternal God but creations or inferior emanations. Arius, a priest in
Alexandria in the 4th century, taught that “there was a time when the Son was
not,” proposing that the Son was the first and greatest creature but not equal
to the Father. To an Arian, calling Jesus divine meant only that he
was godlike or heavenly, not that he literally shared the Father’s infinite
essence. This view was motivated in part by a desire to protect God’s
uniqueness: how could God be one if the Son is also God? Would that not make
two gods, or imply that God begot a second divine being? Arius also pointed to
passages of Scripture where Jesus seems subordinate to the Father (for example,
calling the Father “greater than I,” or appearing limited in knowledge or power
during his earthly life). The Arian error, however, stems from a fundamental
misunderstanding of what it means for the Son to be “begotten” of the Father.
Arius conceived of divine begetting on the model of creaturely causation – as
if the Father produced the Son as an external effect or work, thereby creating
a second, lesser deity. Aquinas directly addresses this mistake: if
“procession” in God were like an effect proceeding from a cause, then indeed
the Arians would be right that the Son and Spirit are creatures, not true God (Aquinas)
(Aquinas).
But divine procession is nothing like creation. The Son’s generation is not
an external act of God making something outside Himself; it is an internal
act of self-communication, as described above. “Careful examination
shows that [Arius and Sabellius] took procession as meaning an outward act…
neither of them affirms procession as existing in God Himself,” Aquinas notes (Aquinas).
For Arius, the Father begetting the Son was like a craftsman constructing a
masterpiece – an act that results in a product separate from the agent.
Thomistic theology flatly rejects this scenario. The Father’s begetting of the
Son happens within God’s own being, not as an act upon an external
object or matter (Aquinas)
(Aquinas).
Therefore it does not yield another, separate being; it yields a person who is
intrinsic to God’s eternal life.
The implications of this for the Son’s divinity are enormous. Because the
Son is begotten inside the infinite Godhead, what He receives from the
Father is the divine essence itself, not a copy or a lesser nature.
The Father gives all that He is to the Son (minus the personal identity of
being “Father”). Nothing of the Father’s divinity is held back or only
partially communicated. As Aquinas says, in God “the divine nature is
communicated by every procession which is not outward” (Aquinas).
Thus the Son is consubstantial with the Father – “one in
being” or of the same substance. Christian tradition, beginning with the
Council of Nicaea (AD 325), forcefully affirmed against Arius that the Son is true
God from true God. Scripture itself leaves no room for the idea of Jesus
as a semi-divine creature. St. John writes unequivocally: “In the beginning was
the Word…and the Word was God” (John 1:1). St. Paul says of
Christ, “In him the whole fullness of deity dwells bodily” (Colossians 2:9).
Aquinas cites St. John’s First Epistle: “that we may be in His true Son, He
is the true God and eternal life” (1 John 5:20) (Aquinas)
(Aquinas).
Likewise, he notes that because Christians are temples of the Holy Spirit, and
only God can have a temple, the Holy Spirit must be truly God (cf. 1
Corinthians 6:19) (Aquinas).
These biblical testimonies align perfectly with the Thomistic metaphysical
insight: if the Son were a creature, not sharing the very being of the Father,
Christianity would indeed be guilty of worshipping a lesser god alongside the
Supreme God – a form of polytheism or idolatry. But because the Son is
of one essence with the Father, when we honor the Son we are honoring the one
God.
Arian objections often cite Jesus’ human limitations in the Gospels or his
filial language (“the Father is greater than I,” John 14:28) as proof of
inequality. Here we must remember the distinction between Christ’s divine
nature and his assumed human nature. Aquinas and orthodox theology teach that
the Son, in the Incarnation, took on a complete human nature (body and soul).
In that human nature, the Son could say the Father is greater, could experience
suffering and ignorance, etc., without detracting from His divine nature. The
Arians, lacking the later Christological clarifications, conflated Christ’s
humanity with his divinity and thus misunderstood those passages. Properly
understood, whenever Scripture speaks of the Son as less than the Father, it
refers to the Son in his role as a man and mediator, not to his eternal divine
essence. As God, the Son is equal to the Father; as man, the Son is subordinate
to the Father (and even to the Spirit, as the Spirit led Jesus in his
ministry). The Athanasian Creed succinctly states: “Equal to the Father, as
touching his Godhead: less than the Father, as touching his Manhood.”
Another Arian concern is that asserting two divine persons (Father and Son)
compromises God’s unity. But this is addressed by the principle already
elaborated: the Father and Son are distinct as persons (relative to
each other) but one in being. They are not two gods, but one God,
because there is a single divine nature. Their relationship can be understood
by analogy to a thought in the mind: my thought is distinct from me in a sense
(I can differentiate “I” and “the idea I have conceived”), yet it is not an external
thing apart from me – it is an inward expression of myself. So the Son is the
Father’s own self-expression, not a second god floating outside the Father.
Aquinas goes even further to ensure we do not think of Father and Son as two
separate entities composing God: since the divine essence is simple,
the Father is that essence and the Son is that essence; they
are numerically one God. The only distinction is the relational one of origin
(Father as origin, Son as from the origin). Remove that relational opposition,
and no distinction remains (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)).
Therefore, classical Trinitarianism is actually the strongest safeguard against
both Arian subordinationism and crude polytheism. It refuses to divide the
substance of God in any way. We do not have a hierarchy of greater and
lesser gods; we have a communion of coequal persons each of
whom is the one infinite God. Arius’s mistake was thinking that
calling the Son “God” would violate monotheism unless the Son were demoted to a
creature. In truth, it would violate monotheism only if the Son were a separate
being alongside the Father. But since Father and Son share the same being,
the unity of God is preserved. Medieval theologians captured this by saying the
Father and Son are “distinct in person but not distinct in nature.”
We can conclude, then, that the Arian objection fails once one understands
Aquinas’s insight that the Son’s generation is an internal procession
that leaves God’s unity intact. The Son is eternally begotten, not made,
and this begetting is an outpouring of the Father’s very substance, not the
production of a new lesser substance. Far from being an affront to reason, the
orthodox doctrine satisfies both the demands of Scripture and the metaphysical
principle that God, as the highest perfection, can communicate His entire being
without loss. The Father, in knowing and loving Himself, eternally generates
the Son and Spirates the Spirit, and in so doing He does not multiply gods but
manifests the richness of life within the one Godhead.
Answering Islamic Objections: One God in Three Persons
The Islamic faith, fiercely monotheistic, has from its inception rejected
the Trinity as incompatible with the oneness of God (tawhid). The
Qur’an insists that God (Allah) has no partners or equals, and it explicitly
repudiates the idea that God has a “son.” To Muslims, the Christian confession
of Father, Son, and Holy Spirit often sounds like tri-theism – as though Christians
worship three separate gods or associate others with God (shirk, the
gravest sin in Islam). Islamic theologians historically have argued that the
Trinity either introduces parts into God or compromises His unity and
simplicity (
Trinity > Judaic and Islamic Objections (Stanford Encyclopedia of
Philosophy) ). For example, the medieval philosopher Al-Kindi interpreted
the Trinity to mean three divine individuals each composed of the one divine
essence plus a distinguishing characteristic, and he rightly noted that any
such composition would mean those individuals are not eternal or
self-sufficient (
Trinity > Judaic and Islamic Objections (Stanford Encyclopedia of
Philosophy) ). The Muslim critique thus poses a serious question: does the
doctrine of the Trinity maintain the absolute unity and uniqueness
of God, or does it make God into a committee of three? From a Thomistic perspective,
we assert firmly that Trinitarian doctrine, properly understood, upholds God’s
oneness in the strongest possible way. The unity of essence in the
Trinity means that Christians no less than Muslims affirm there is exactly one
God, one ultimate being who alone is worthy of worship. We do not
believe in three separate gods, nor in one God who is split into three pieces
or modes. As the Catechism of the Catholic Church explains in continuity with
ancient councils: “Father, Son, Holy Spirit are not three principles of
creation but one principle,” because “each of the persons is that supreme
reality, viz., the divine substance, essence or nature” (CCC 258). The threeness
of God lies only on the personal level (the relationships of Father, Son,
Spirit); it does not multiply the godhead itself. Thus, in Christian
understanding, the Father, Son, and Spirit are inseparable in what
they are and what they do. There is a beautiful line in the Gospel of John that
illuminates this unity: Jesus says of the Father, “I and the Father are one”
(John 10:30). The Greek word for “one” here is neuter (hen), indicating unity
of nature or essence, not merely agreement of will. Yet Jesus also prays to the
Father and speaks to Him – showing He is personally distinct. Early Christian
theologians captured this mystery with the formula that the Son is “one in
essence” (homoousios) with the Father, even though He is another
as a person. The Qur’an appears to misconstrue the Trinity in one verse as
“God, Jesus, and Mary” – which is indeed a tritheistic and absurd formulation
rejected by all orthodox Christians. In reality, Mary is a creature and not
part of the Godhead, and Jesus is not a separate god beside the Father but the
incarnate Word of the one God. So the Trinity is not “God plus two
others,” as some Muslim polemics imagine, but rather one God in three personal
self-distinctions.
Aquinas and other scholastics were well aware of the philosophical objection
that any real plurality in God seems to imply composition and thus negate
simplicity. The answer they give is subtle: the plurality of persons in God is not
like any other plurality we know. In creatures, multiple individuals of one
species (e.g. three human beings) means three separate substances, each with
its own divided portion of a common nature. But God is not a species that can
have multiple members; God is infinite being. There cannot be three separate
infinite beings, since each, to be distinct, would have to lack something the
others have – which is impossible if each is truly God. Therefore, Christianity
does not posit three parallel gods. Rather, the three persons share
the one infinite being, like three candles all lit from the same flame (an
analogy offered by medieval theologians to illustrate how one nature can be
wholly in more than one person – though every analogy has limits). Another way
to put it: when we say the Father is God, the Son is God, the Spirit is God, we
do not mean there are three instances of deity as a genus or class. We
mean the one Godhead is simultaneously Father, Son, and Spirit. This admittedly
transcends full human comprehension, but it is not logically incoherent. It
would be incoherent only if we said something like “three persons are one
person” or “three beings are one being” in the same respect. But we do not say
that. We say one being is three persons. Personhood and being are different
categories: being answers the question “what are you?” whereas person
answers “who are you?”. In created things, normally one being equals one person
(e.g. a human being is one person). But God is radically different – His mode
of being is unique and surpasses created analogies. It is not illogical
that the ultimate reality might exist in a way that is one-and-plural on
different planes (an analogy from geometry: three distinct coordinates can
define one single point in space – here dimensionally different parameters
coincide in one point). In God’s case, the “parameters” are not dimensions but
the personal relations of origin that we discussed. The Father, Son, and Holy
Spirit are distinct as origin, begotten, and proceeding, yet they coincide in
the one divine essence.
Islamic objectors also charge that Trinity violates simplicity by
effectively saying God has “parts” (the persons) or accidents (the relations).
The Thomistic answer is that the persons are not parts of God, nor are
the relations accidents inhering in a subject. In God, there cannot be
accidents or separable parts at all (Trinity
and Simplicity — The Reformed Classicalist) (Trinity
and Simplicity — The Reformed Classicalist). The persons are best
understood as subsistent relations – each person is the one
God under a distinct relational aspect. This does not make the relations
unreal; on the contrary, Father, Son, and Spirit are really
distinct, but their distinction lies wholly in the relations of
origin, not in any composition of the divine essence (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)) (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)). To
a Muslim accustomed to thinking of God as a single-personal being, this is
admittedly a foreign concept. The idea of relation as something that
can subsist in and as the very being of God comes from Christian reflection on
revelation aided by Aristotle’s philosophy of relations. Aquinas notes that in
creatures, relations are typically accidents (e.g. the relation of fatherhood
in a human is an accidental feature, not the substance of the man). But in God,
relations being accidents would indeed compromise simplicity.
Therefore, he concludes that in God the relations must be identical
with the divine essence (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)).
That is, God’s very nature is to be Father knowing Himself and Word known, and
Love proceeding from both. The relations are “what God is” just as much as the
divine attributes are – they are only different in relation to each other, not
in relation to God’s essence (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)).
This profound point shows that the Trinity does not introduce foreign
elements into God; it is simply God’s one perfect being, considered in its
self-relatedness. As the Council of Florence taught, all such relations are
within the single divine substance, essence, or nature, which remains
utterly one (Paragraph
2. The Father - The Holy See). If a Muslim interlocutor insists that God cannot
have any internal distinctions at all, one might respond that even in Islamic
theology God is described by many names or attributes (merciful, just, living,
powerful, etc.). How can an absolutely simple being have multiple attributes?
Classical Muslim theologians typically say those many names are all really one
in God and only seem multiple to our finite minds – a view not far from the
Christian understanding that in God’s simplicity, love = wisdom = power =
being, etc. The difference with the Trinity is that the “three” are not
attributes or acts directed outward, but personal relations of origin. Still,
the principle is similar: plurality in God (of whatever sort) does not
necessarily violate unity if that plurality does not compose or divide the divine
essence. In Christianity’s claim, the “Threeness” is a relational plurality
that actually requires the unity of essence as its underlying context. If there
were three separate gods, they could not be Father, Son, and Spirit in relation
– they would be three unrelated absolutes, which is not the Trinity at all.
Paradoxically, only if God is one can He exist as a Trinity of persons, since
only with one shared essence can the persons coinhere and love one another in
total self-gift. If God were a solitary monad (as in strict Unitarian
theology), God could not have the attribute of interpersonal love “built-in”
from eternity – He would need creation to have something to love. The Christian
vision of God as inherently relational Love (cf. 1 John 4:8, “God is love”)
thus preserves God’s self-sufficiency (the three love one another perfectly,
lacking nothing) and casts a new light on divine unity: it is not the unity of
a lonely being but the unity of a communion. This does not
convince by logical syllogism – Muslims and Christians ultimately have
differing authorities and starting premises – but it shows that the doctrine of
the Trinity is internally coherent and does not amount to tri-theism.
No one in Trinitarian theology is asserting “3 gods = 1 god” which would indeed
be nonsense. We assert one God in three persons, which is
mysterious but coherent when we unpack the meanings of “person” and “essence.”
Aquinas even addresses a common-sense worry: if each of the three is God, do we
have three Gods? He answers no, because in God “what is” (the nature) is
identical with “who is” (the person) only when considering each person singly (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)).
But since the nature is one, there are not three natures to yield three Gods.
The term “God” can be predicated of Father, Son, Spirit in the singular, not as
a plural count of gods (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)).
Christian thinkers sometimes use the analogy of a triangle: it has three
corners, yet it is one triangle. The corners are distinct, but they are not
three separate triangles; each corner is a “manifestation” of the whole
triangle’s essence under a different aspect. The Trinity is infinitely greater
and more mysterious than any geometric shape, but the analogy hints that
threeness and oneness need not be incompatible in principle.
Finally, Muslims argue that God revealing Himself as a Trinity introduces a
“mystery” that offends the clarity of pure monotheism. Islam prides itself on a
simple, comprehensible creed: There is no god but God; Muhammad is His
messenger. Christianity agrees there is only one God, but adds richness to
that statement by confessing Father, Son, Holy Spirit within the Godhead –
something we could not know without God telling us. Aquinas concedes that the
Trinity is not discoverable by natural reason; it is known only by divine
revelation (through Christ and the apostolic witness) (
Trinity > Judaic and Islamic Objections (Stanford Encyclopedia of
Philosophy) ) (
Trinity > Judaic and Islamic Objections (Stanford Encyclopedia of
Philosophy) ). But once revealed, it is fitting and reasonable in
the highest sense – it does not violate reason, it transcends
it. If one believes God is an infinite, transcendent reality, it is not
surprising that aspects of His inner life exceed the grasp of finite human
logic. Rather than contradicting monotheism, the revealed mystery of the
Trinity elevates our understanding of the unity of God,
showing it to be a living, fruitful unity, not a barren singularity. Many
Muslim thinkers hold that God’s oneness is so absolute that even attributes
like knowledge and will are only nominally distinct (lest God appear composed).
Ironically, this can make God almost impersonal – a pure will with no inherent
relationality or love. The Trinity, in contrast, presents a God who is super-personal
– not less than personal, but a communion of Persons. This is a unity that is
dynamic and fecund (the Father eternally generates the Son, and the Father and
Son breathe forth the Spirit), yet it remains one being. Christianity
thus maintains, just as firmly as Islam, that there is only one God, even as it
invites people to know this one God more intimately as Father, through the Son,
in the Holy Spirit. The charge of polytheism only sticks if one misunderstands
what the doctrine actually teaches. When correctly understood via the Thomistic
metaphysical distinctions, the Trinity emerges as a profound mystery that in no
way compromises the oneness, simplicity, or sovereignty of God.
Mystery and Rationality: Triune Truth as Accessible to Reason and
Faith
To human reason alone, the inner life of God as Holy Trinity would have
remained unknown. Aquinas teaches that the truth of the Trinity “surpasses the
capacity of human reason,” and thus had to be revealed to us (
Trinity > Judaic and Islamic Objections (Stanford Encyclopedia of
Philosophy) ). Nevertheless, he also insists that this mystery, once
revealed, is not irrational or absurd. The Trinity is a mystery,
meaning it is a truth that is inexhaustibly deep and cannot be comprehended
fully by our finite minds. But a mystery in the theological sense is not a
logical contradiction; it is a reality whose inner workings we cannot
completely grasp, yet we can approach by analogy and be confident it contains
no error or inconsistency. In fact, part of the task of Christian theology
(especially in the Thomistic tradition) is to show that mysteries of faith are
at least negative mysteries – they do not force us to believe
something contradictory, even if we cannot imagine exactly how it is
so. For example, the statement “one God in three persons” sounds paradoxical,
but through careful definitions of one (essence) and three
(persons), we see it is not the same kind of “one” and “three,” and thus not a
direct contradiction. Reason can affirm that the doctrine is internally
coherent (when terms are properly understood) (Ye
Olde Trinity Diagram: The Shield of Faith – Trinities) (Ye
Olde Trinity Diagram: The Shield of Faith – Trinities). Reason can also
illuminate fitting analogies (mind-word-love, etc.) to partially illumine the
mystery. Yet reason will also humbly acknowledge its limits: we cannot prove
the Trinity by logical deduction, nor can we fully comprehend how one
divine essence is entirely possessed by three distinct persons. We rely on
God’s self-revelation in Christ and the Holy Spirit’s guidance of the Church to
know that the Trinity is real.
Aquinas famously said that in this life we are like wayfarers who know God
“as in a mirror, dimly” – our concepts and analogies fall short of the divine
reality (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)) (Aquinas).
But he also believed that our analogical knowledge is true as far as it goes.
For instance, knowing God as intellect and will (by analogy
to our soul) is a true insight, even if God’s intellect and will are one and
the same in a way ours are not. Likewise, understanding the Son as the Word
of the Father and the Spirit as the Love of Father and Son
gives us a genuine, if analogical, glimpse into why God is triune. It shows
that the Trinity, rather than being an arbitrary conundrum, actually
corresponds to God’s nature as the highest intellect and highest love. It would
be unworthy of God to be a solitary thinker without a Word, or a lover without
an eternal Love. The Trinity reveals that God is not a solitary distant deity,
but an eternal communion of truth and love. Human reason, reflecting on this,
can see a profound beauty and consistency in the doctrine. It seems
contradictory only if we impose on God the limitations of created beings (e.g.
assuming that three persons must mean three separate beings, which
holds for finite creatures but not for the infinite Creator). Once we allow
that God is in a category of His own (“His ways are above our ways”), the
Trinity can be accepted as a unique mode of existence that has no exact
parallel elsewhere – and yet leaves traces in creation (like the image of God
in man as rational and loving, which points to a triune original).
It’s worth noting that Christian theology does not ask us to check our
rationality at the door; rather, it asks us to enlarge our intellect by faith. Faith
and reason work together (fides et ratio). The
mystery of the Trinity is first received by faith in God’s revelation. But
faith seeks understanding, and so we use reason to explore and clarify what we
can. Aquinas argued that while natural reason cannot demonstrate
the Trinity, it can show that the Trinity is fitting (conveniens) and
free from contradiction (
Trinity > Judaic and Islamic Objections (Stanford Encyclopedia of
Philosophy) ). For example, he demonstrates that having only two
processions (intellect and will) in God avoids an infinite regress of persons (Aquinas),
and that because “whatever is in God is God,” an internal procession implies
consubstantiality (Aquinas).
These rational reflections reassure the believer that the doctrine hangs
together and does not ask one to believe “three equals one” in a simplistic
way. In a sense, reason acts as the handmaiden of faith here:
it tidies up our language, refutes misunderstandings, and draws out
implications, even as the core truth remains a gift beyond unaided reason.
Thus, the Trinity is mysterious but not absurd. We call it
a rational mystery – not in the sense that reason can
exhaustively explain it, but in the sense that reason can see the mystery’s
contours and affirm its possibility. This stands in contrast to a square
circle, which is a true contradiction that no amount of higher
understanding can salvage. The Trinity is not like that; it’s more like light
split through a prism – appearing as three colors but coming from one pure
light. To a mind that only knew monochrome, the spectrum might seem impossible,
yet deeper insight shows it to be real and consistent. Similarly, the prism of
divine revelation shows us plurality in the one divine Light. Aquinas’
metaphysics – with its concepts of simplicity, relation, substance, personhood
– acts as a kind of intellectual prism that helps us make sense of what we are
seeing, assuring us it is not nonsense or a trick, but the real nature of the
divine being, however ineffable.
Conclusion
From a Thomistic perspective, the doctrine of the Trinity emerges as a
sublime harmony of unity and plurality in God. We have seen that by deploying
the concepts of essence, procession, and relation, St. Thomas Aquinas provides
a framework in which Father, Son, and Holy Spirit are understood as distinct persons
precisely through their relations of origin, and yet as one being
by virtue of the one simple divine essence they each are. Divine simplicity is
upheld: God is not composed of parts or lesser elements, and the three persons
do not partition the Godhead. Rather, as Aquinas showed, the divine relations are
the divine essence in its relational mode, so that “in God, the essence is not
really distinct from the person” (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39)) and
“there are one essence and three persons” (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39))
without contradiction. We addressed the Arian objection by clarifying that the
Son’s generation is an internal communication of the fullness of deity, not the
creation of a secondary god. The Son is begotten, not made,
eternally receiving the one identical divine nature from the Father, and thus
He is fully God, co-equal with the Father – a truth rooted in Scripture and
articulated at Nicaea against Arius’s subordinationism. We then engaged the
Islamic objection, affirming that the Trinity does not compromise God’s
oneness: it is a doctrine of one God in three persons, not
three gods. Thomistic analysis underscores that the unity of God’s essence is
absolute, and the distinctions of person are real but relational. Hence, the
Trinity does not entail polytheism or any division in the divine substance. It
remains a profound mystery, yes, but one that is coherent in itself and fitting
to the nature of a God who is Love and Word eternally. The relational
opposition of Father, Son, and Spirit allows for personal differentiation
without breaking unity – “everything (in God) is one where there is no
opposition of relationship” (Catechism
of the Catholic Church | Catholic Culture).
In the end, the Trinity invites us to adore a God whose inner life is an
everlasting communion of love, knowledge, and gift. Aquinas would remind us
that our concepts can only go so far; we navigate between the
errors of tritheism and modalism by sticking closely to the language of one
essence and three relational subsistences, even if we cannot
imagine a created example of such a thing. The seeming paradox of the Trinity
humbles our intellect, but does not humiliate it – rather, it elevates reason
to consider realities above its natural reach. In the Thomistic vision, faith
perfects reason, and reason, in turn, finds the Trinity to be not an enemy but
a luminous mystery that both satisfies and surpasses our deepest philosophical
longings. We conclude that the doctrine of the Trinity, defended on
metaphysical grounds by thinkers like Aquinas, stands as a internally
consistent and theologically compelling portrayal of the one true God. It is
not irrational – indeed, it would be irrational to claim exhaustive
understanding of an infinite God. The Trinity is a mystery, but one accessible
to reasoned insight and entirely worthy of belief. In the final analysis, the
Thomistic approach helps us echo with understanding what Christians have
confessed for centuries: Glory be to the Father, and to the Son, and to the
Holy Spirit – the Trinity in Unity and Unity in Trinity, one God forever and
ever. (SUMMA
THEOLOGIAE: The persons in relation to the essence (Prima Pars, Q. 39))
