Theme: Jesus Christ will certainly bring judgement upon false teachers, contend for the faith

Why does Jude quote from 1 Enoch?
In Jude 14-15 there is a quotation taken directly from 1 Enoch 1.9. 1 Enoch is a pseudepigraphical work attributed to the Enoch of Genesis 5:18-24 but not written by him. Apart from the Ethiopian Church, it is not recognised as canonical Scripture by any Christian group. Most probably Jude quoted from this book because he believed that the selected portion clearly represented God’s truth. Why he didn’t refer to another part of Scripture instead is not clear. It may be that the individuals who infiltrated the church (Jude 4) had a high regard for the book and therefore Jude uses their own ammunition against them. However there is no hint that Jude merely regards this text as authoritative for his opponents and the letter is directed to the church as a whole, not to the heretics.

Augustine’s comments on the matter are worth repeating:
Let us omit, then, the fables of those scriptures which are called apocryphal, because their obscure origin was unknown to the fathers from whom the authority of the true Scriptures has been transmitted to us by a most certain and well-ascertained succession. For though there is some truth in these apocryphal writings, yet they contain so many false statements, that they have no canonical authority. We cannot deny that Enoch, the seventh from Adam, left some divine writings, for this is asserted by the Apostle Jude in his canonical epistle. But it is not without reason that these writings have no place in that canon of Scripture which was preserved in the temple of the Hebrew people by the diligence of successive priests; for their antiquity brought them under suspicion, and it was impossible to ascertain whether these were his genuine writings, and they were not brought forward as genuine by the persons who were found to have carefully preserved the canonical books by a successive transmission. So that the writings which are produced under his name, and which contain these fables about the giants, saying that their fathers were not men, are properly judged by prudent men to be not genuine; just as many writings are produced by heretics under the names both of other prophets, and more recently, under the names of the apostles, all of which, after careful examination, have been set apart from canonical authority under the title of Apocrypha.
(City of God 15.23)