Because he quotes verbatim from it, and elsewhere uses language and concepts undeniably from the book. The same goes for the Assumption of Moses. The author of Jude was clearly familiar with both books. Also 1 Enoch was one of the most influential books of the period and was quoted as scripture by other early Christian writers. It also should be pointed out that the passage quoted directly by the author of Jude is extant in pre-Christian manuscripts of 1 Enoch in the Dead Sea Scrolls. The only other book circulating under the name of Enoch (2 Enoch) lacks the parallels with Jude and was clearly written much later, quite possibly after Jude was composed (that is, in the second century AD; it is extant only in Old Church Slavonic and in a Coptic fragment, and the earliest evidence of it appears in Clement of Alexandria and Origen).
Jude 3-4: " I found it necessary to write urging you to contend for the faith that was once for all delivered to the saints (tè hapax paradotheise tois hagiois pistei)"
Assumption of Moses 4:8: "But the two tribes shall remain in the faith first laid down for them (permanebunt in praeposita fide sua)".
Jude 6: "And angels who did not keep their own domain, but abandoned (apolipontas) their proper abode (to idion oikètèrion), he has kept (tetèrèken) in eternal (aidiois) bonds (desmois) under dense darkness (hupo zophon) for the judgment of the great day (eis krisin megalès hèmeras)".
1 Enoch 12:4: "Go and make known to the Watchers of heaven who have abandoned (apolipontes) the high heaven, the holy eternal place, and have defiled themselves with women."
1 Enoch 15:3, 7: "For what reason have you abandoned (apelipete) the high, holy, and eternal heaven; and slept with women and defiled yourselves with the daughters of the people.... I did not make wives for you, for the abode (hè katoikèsis) of spiritual beings of heaven is heaven".
1 Enoch 10:4-6: "Go, Raphael, and bind (dèson) Asael hand and foot and throw him into the darkness (skotos) and let him dwell there for eternity (eis tous aiònas).... Cover up his face in order that he may not see light, in order that he may be sent into the fire on the great day of judgment (en tèi hèmerai tès megalès tès kriseós)".
1 Enoch 10:11-12: "Go, Michael, and bind (dèson) Shemihazah and the others with him, who have fornicated with the daughters of men, that they will die together with him in all their defilement.... Bind them (déson autous) for seventy generations underneath the rocks of the ground until the day of their judgment (hèmeras kriseòs) and consummation until the eternal (tou aiònos tòn aiònòn) judgment (krima) is concluded".
1 Enoch 14:5: "It has been decreed to bind you (dèsai humas) in bonds (en tois desmois) in the earth for all the days of eternity (eis pasas tas geneas tou aiònos)".
Jude 9: " But even the archangel Michael, when he was disputing with the Devil about the body of Moses, did not himself dare to condemn him for slander but said, 'The Lord rebuke you!' "
Assumption of Moses (lost conclusion): The only surviving manuscript is missing its ending but it certainly contained an account of Moses' death, burial, and assumption of his soul to heaven. According to Gelasius, it contained a story in which the archangel Michael disputed with the Devil: " In the book of the Assumption of Moses, the archangel Michael in a discussion with the Devil says: 'For by his Holy Spirit all of us have been created' and further he says 'God's spirit went forth from his face, and the world came into being' " (Historia Ecclesiastica 2.21.7). His knowledge of the book was accurate since the same passage also quotes from Assumption of Moses 1:14. Origen affirmed that this was "a book which the apostle Jude mentions in his epistle" (De Principiis, 3.2.1). Didymus Caecus also noted that some people "take exception to the present epistle and object to the Assumption of Moses, on account of that place where the archangel 's word to the Devil concerning the body of Moses is indicated" (In Epistula Judae Ennaratio). The lost story of Moses' burial and assumption from the Assumption of Moses was also probably summarized by Clement of Alexandria: "Joshua son of Nun saw Moses double when he was taken up: one Moses with the angels and another one in the mountains honored with burial in their ravines. And Joshua saw this spectacle below being elevated by the Spirit along with Caleb" (Stromateis 6.15). There are parallels to other extracanonical stories about the death and burial of the patriarchs:
Life of Adam and Eve 39:1 (Latin recension): "Then Seth said to the beast : ' May the Lord God rebuke you! Stop, be quiet, close your mouth, cursed enemy of truth, chaotic destroyer' ".
Life of Adam and Eve 37:4-6, 40:1-6 (Greek recension): "The Lord of all, sitting on his holy throne, stretched out his hands and took Adam and handed him over to the archangel Michael saying to him, 'Take him up to Paradise, to the third heaven, and leave him there until that great and fearful day which I am about to establish for the world.' And the archangel Michael took Adam and brought him away and left him, just as God told him at the pardoning of Adam....Then he spoke to the archangel Michael , 'Go into Paradise in the third heaven and bring me three cloths of linen and silk.' And God said to Michael , Gabriel, Uriel, and Raphael, 'Cover Adam's body with the cloths and bring oil from the oil of fragrance and pour it on him....Then the angels took up the body and set it on the rock, until the time his father died, and both were buried according to the command of God in the regions of Paradise in the place from which God had found the dust".
Testament of Abraham 7:15-16: "And Michael said to Abraham, 'Your son Isaac has spoken the truth, for you are like the sun, and you will be taken up into the heavens while your body remains on the earth until even thousand ages are fulfilled. For then all flesh will be raised' ".
Testament of Abraham 20:10-12, 14: "And immediately the archangel Michael stood beside him with multitudes of angels, and they bore his precious soul in their hands in divinely woven linen. And they tended the body of the righteous Abraham with divine ointments and perfumes until the third day after his death. And they buried him in the promised land at the oak of Mamre, while the angels escorted his precious soul and they set it down for the worship of the God and Father .... who came speaking thus, 'Take, then, my friend Abraham into Paradise where there are the tents of my righteous ones and where the mansions of my holy ones, Isaac and Jacob, are in his bosom, where there is no toil, no grief, no moaning, but peace and exultation and endless life".
Jude 12-13: "They are clouds blown along by the wind without giving rain (nephelai anudroi hupo anemòn parapheromenai), autumnal trees bearing no fruit (dendra phthinopòrina akarpa), dead twice over and uprooted, wild waves of the sea (kumata agria thalassès) casting up the foam of their abominations, wandering stars (asteres planètai) for whom the black darkness (ho zophos tou skotous) has been reserved (tetèrètai) forever (eis aiòna)".
1 Enoch 80:2-6: "In the days of the sinners the years will grow shorter ... everything on the earth will change and will not appear at their times, the rain will be withheld, and the sky will retain it. At those times, the fruit of the earth will be late and will not grow at its normal time, and the fruit of the trees will be withheld at its proper time. The moon will change its order and will not appear at its proper time ... Many heads of stars will wander from their command and will change their ways and actions and will not appear at the times prescribed for them".
1 Enoch 101:2-4: "If he closes the windows of heaven, and withholds the dew and the rain from descending because of you, what will you do? Why do you speak with your mouth proud and hard things against his majesty? You will have no peace. Look at the captains who sail the sea! Their ships are shaken by wave and storm".
1 Enoch 18:14-16: "This place is the prison (desmòtèrion) for the stars and the hosts of heaven (astrois kai tais dunamesin tou ouranou).... They transgressed the command (hoi parabantes prostagma) of the Lord in the beginning of their rising, for they did not come out in their appointed times (ouk exèlthan en tois kairois autòn). And he was angry with them and bound them (edèsen autous) until the time of the consummation of their sins, that is for ten thousand years".
1 Enoch 21:6: "These are the stars of heaven that transgressed the command (tòn asteròn tou ouranou hoi parabantes tèn epitagèn) of the Lord; they have been bound (edethèsan) here until ten thousand years are fulfilled, that is, the time of their sins".
1 Enoch 88:1-3: "He seized that first star that had fallen from heaven (astera ek tou ouranou peptòkota), and he bound it (edèsen auton) by its hands and feet and threw it into an abyss (eis pharaggas), and that abyss (pharagx) was narrow and deep and desolate and dark (skoteinè).... Behold, one of those four who had come from heaven hurled stones from heaven and gathered and took all the great stars (panta asteras megalous), whose organs were like the organs of horses, and bound (edèsen) all of them by their hands and their feet, and threw them into an abyss (eis pharaggas) of the earth".
Jude 14-16: " Enoch, the seventh from Adam, prophesied about them: 'Behold, the Lord is coming with myriads upon myriads of his holy ones (ecce venit Dominus in sanctis milibus suis) to execute judgment on all (facere judicium contra omnes), and to convict all of them of all the ungodly deeds they have done (arguere omnes impios de omnibus operibus) in an ungodly (impie) way, and for all the harsh words ungodly sinners have spoken against him (de omnibus duris quae locuti sunt contra eum peccatores impii)'. These ones are grumblers (quaerulosi sunt ) and faultfinders; they follow their own evil desires; their mouths utter proud things ( os illorum loquitur superba ) and they show partiality to persons ( mirantes personas ) for the sake of profit ( quaestus causa )".
1 Enoch 1:9: "Behold, he comes with myriads of his holy ones (ecce venit cum multis milibus sanctorum suorum) to execute judgment on all (facere judicium de omnibus), and to convict all flesh for all the wicked deeds they have committed (arguere omnen carnem de omnibus factis impiorum quae fecerunt), and for all the proud and harsh words that the wicked sinners have spoken against him (de omnibus verbis impiis quae locuti sunt contra eum peccatores)".
1 Enoch 5:4: "You have not been long-suffering and you have not done the commandments of the Lord, but you have transgressed and spoken proud and harsh words with your unclean mouths against his majesty (locutus superbus et dura verba cum immundus ore contra majestatem)".
1 Enoch 101:2-3: "Why do you utter with your mouth proud and harsh things against his majesty (quid cum ore superbus et dura quae contra majestatem loqui)?"
Assumption of Moses 5:5: "For the scholars who will be their teachers in those times will show partiality to persons ( mirantes personas ) who please them and they accept gifts (acceptiones munerum)".
Assumption of Moses 7:7-9: " They will be grumblers (quaerulosi erunt), liars, hiding themselves lest they be recognized as ungodly (impii)...And their hands and minds will deal with impurities, and their mouth will speak enormities (os eorum loquetur ingentia)".
