Saint Jude the Apostle was one of the Twelve Apostles of Jesus but is not to be confused with Judas Iscariot, the apostle who betrayed Jesus prior to his crucifixion. He is generally identified with Thaddeus, and is also called Jude of James and is sometimes identified with Jude, the brother of Jesus. His authorship of the Epistle of Jude is uncertain due to the common use of his name in the Bible and ambiguous relational connections. By Georges de La Tour (1624-1650) Image Source
The Epistle of Jude is the next-to-last book of the New Testament and is attributed to "Jude, the servant of Jesus Christ, and brother of James". With one chapter of 25 verses, Jude is tied with Obadiah, Philemon, 2 John and 3 John for books with one chapter in the Bible.
After a salutation (verses 1-3) the book begins by discussing the sin and doom of "ungodly men, turning the grace of our God into lasciviousness, and denying the only Lord God, and our Lord Jesus Christ" (verse 4) and provides historical examples of the judgement of unbelievers in Israel (5), angels (6) and Sodom and Gomorrah (7). Then unbelievers during the time of Jude are described (8-16) including slanderous speech (8-10) and character (11-13). Finally, their destruction is prophesied (14-16).
Then Jude shifts to call on believers to set themselves apart from those "who should walk after their own ungodly lusts. These be they who separate themselves, sensual, having not the Spirit." (18-19) and persevere keeping "yourselves in the love of God, looking for the mercy of our Lord Jesus Christ unto eternal life" (verses 17-23). It ends with praise "To the only wise God our Saviour, be glory and majesty, dominion and power, both now and ever. Amen" (25).