14 And Enoch also, the seventh from Adam, prophesied of these, saying, Behold, the Lord cometh with ten thousands of his saints,
15 To execute judgment upon all, and to convince all that are ungodly among them of all their ungodly deeds which they have ungodly committed, and of all their hard speeches which ungodly sinners have spoken against him.
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Is Jude wrong?
That said absolutely nothing about demons being the spirits of dead nephilim.
The very fact that Jude spoke of Enoch and used the language in the Book of Enoch (i.e., "[T]he angels who did not keep their positions of authority but abandoned their proper dwelling—these he has kept in darkness, bound with everlasting chains for judgment on the great Day") means that he was incorporating it by reference.
In fact, that one quotation from Jude destroys the hypothesis of those who say the "Sons of God" in Genesis 6 are the sons of Seth or merely great men (royalty).