The Niceno-Constantinopolitan Creed, derived from the decision of the Council of Nicea of 325 C.E., states the following:
"I believe in one God, the Father almighty, Maker of heaven and earth . . . And in one Lord Jesus Christ, the Son of God, the only-begotten, begotten of the Father before all ages. Light of light; true God of true God; begotten, not made; of one essence with the Father . . . And in the Holy Spirit, the Lord, the Giver of Life, who proceeds from the Father; who with the Father and the Son is worshipped and glorified . . ."
This is the classical definition of Trinitarianism. It must be understood in the light of the alternatives which were offered at that time in history. One was Arianism - the belief that the Logos-Christ was a creature (and which is held by JWs today) - and the other was Modalism. Modalism was the belief that God was one Person who played three roles - that of Father, Son, and Holy Spirit. Modalism can easily be mistaken for Trinitarianism, and people today who are simply contented to say that "Jesus is God" are frequently Modalists without realizing it. Both Modalism and Arianism have the advantage that they are logically consistent within themselves, but neither of these theories takes into consideration all the Scriptural evidence regarding the relationship of Jesus to the Father.
To illustrate the three approaches, let's look at John 1:1 - "In the beginning was the Word, and the Word was with the God, and the Word was God." The Arian says, "the Word was a god" (without the article), meaning that the word was a separate and distinct being from the Father. The Modalist says, "The Word was God," and therefore must have been the Father. But the Trinitarian says, "The Word was God," but not the Father.
Historically, the best way to explain the Trinity is to begin with the Father, and then to show the relationship of the Son and Spirit to the Father. The Son is begotten of the Father, meaning that the Son is generated out of the Father and is therefore not a separate and distinct God. The Spirit is like the breath of the Father's mouth, and is therefore divine like the Father.
In his "Dogmatic Theology, Vol. IV - The Trinity," Francis J. Hall wrote in 1910:
"(a) Several particulars are to be noted. In the first place, the principle of origin in the Trinity is absolutely one, and is seated in the Father. . . . The other divine Persons proceed from Him, but He proceeds from none. Each divine Person is [theos - God], for each possesses the divine essence, and without confusion contains the other two; but the Father is [Autotheos - God himself].
"(b) This introduces the second particular, that neither the Son nor the Holy Spirit is [Autotheos], for each derives His essence - His being as God - from the Father. These two are God in a subordinate MANNER, although not in a subordinate SENSE of the word God. That is, their being God is due to their proceeding from the Father; but they are as truly God as is the Father, because the very essence of the Father is fully and eternally Theirs. They are co-eternal and co-equal with God the Father." (p. 239)
Hall also states: ". . . when the Persons are mentioned together it is the Father alone to whom the name God is applied. This is so because He is the fountain of Deity, and the other two Persons are God because They are derived from Him and participate in His essence. They are indeed given divine titles at times when separately mentioned, lest we should be deceived as to Their co-equality with the Father; but none the less care is taken when the Father is mentioned with Them . . ." (p. 241)
This is not to deny that other Trinitarians refuse to recognize these relationships between the Persons, or to recognize that there is subordination WITHIN the Godhead, and thus revert to a sort of Modalism. But it also means that most JW arguments (coming from the opposite extreme of Arianism) are actually refutations of Modalism rather than the formal Trinitarian doctrine that was worked out in the fourth century.
Once the terms are clearly defined, let all search the Scriptures to determine which theory is most in accord with the Scriptural evidence.