Heidegger's Bible Handbook: Jude: Authorship
1. The Author of the Epistle is the Apostle Jude, not some Bishop of Jerusalem. Grotius’ objections are answered.
The Author of the Epistle is Ἰούδας, Ἰησοῦ Χριστοῦ δοῦλος, ἀδελφὸς δὲ Ἰακώβου, Jude, the servant of Jesus Christ, but brother of James, namely, the son of Alphæus, verse 1. But the Inscription calls him Jude the Ἀπόστολον/Apostle; Tertullians also in de habitu Mulierum, chapter 3, thus cites it: To this it is added that in the book of the Apostle Jude Enoch holds this testimony. It is added that the Apostle Jude is expressly called the brother of James in Mark 6:3; Luke 6:16; Acts 1:13. Hence it comes to be amazed at, that it was able to come into the mind of Grotius, that not the Apostle Jude, but some other Jude, the fifteenth Bishop of Jerusalem, who lived in the times of Hadrian, a little before Bar Kokhba, was the author of the Epistle. For, the fact that he calls himself, not Apostle, but the servant of Christ, does not at all argue that it was not written by the Apostle. For, Saint Paul also sometimes calls himself simply the servant of God, not the Apostle, as in Philippians 1:1, indicating that he wrote, not on his own initiative, but by the commandment of God, and in discharge of his ministry. But it was not necessary for Jude to call himself an Apostle, because in this Epistle he is not so much his intention to testify of the words and deeds of Christ, as simply to admonish his brethren, that they contend for the faith once delivered. Neither does Paul call himself an Apostle in other Epistles, such as those to the Thessalonians and Philemon. Jude does not exclude himself from the Apostles in verse 17, commanding them to remembers the words spoken before ὑπὸ τῶν ἀποστόλων τοῦ Κυρίου ἡμῶν Ἰησοῦ Χριστοῦ, by the Apostles of our Lord Jesus Christ; but rather appeals to the Apostles that had written before him. He cites the words of Peter, 2 Peter 2, just as also Peter cites the Epistles of Paul. Moreover, he is in the list of the Apostles, Matthew 10:3; Mark 3:18, under the name of Λεββαίου/ Lebbæus, which means prudent, and תַּדַּי, Θαδδαίου/Thaddæus, which means confessor.
 See Matthew 10:3; Mark 3:18; Luke 6:15; Acts 1:13.  Emperor Hadrian reigned from 117 to 138.  In response to the Bar Kokhba revolt (132-135), Hadrian sent the Roman army. There were massive casualties on both sides.  2 Peter 3:15, 16.