Parts of Jude are preserved in two early extant papyrus manuscripts (P72 and P78, both of the late third or early fourth century CE). Eusebius, in his Ecclesiastical History (II, xxiii, 25; III, xxv, 2), acknowledged that, because only few writers referred to it, the authenticity of the letter of Jude was doubted. Nevertheless, in the time of Eusebius, many were acquainted with this letter, and communities of believers regularly used it.
Without any clear evidence to the contrary, the opening verse may rightly be accepted as identifying the writer of the letter to have been Jude, the brother of James. Of the two, James the brother of the Lord, was more prominent than Jude and so would have been more widely known. Therefore, by referring to himself as the brother of James, Jude made his identity clear.
Ancient tradition confirms that Jude was of the tribe of Judah in the royal line of David, which agrees with the fact that the family into which Jesus Christ was born had descended from David. In the second century CE, Hegesippus wrote about the grandsons of Jude in connection with an incident during Domitian’s reign, “And there still survived of the Lord’s family the grandsons of Jude, who was said to be His brother, humanly speaking. These were informed against as being of David’s line.” The grandsons were devoted disciples of the Lord Jesus Christ and reportedly told Domitian that Christ’s kingdom was “not of this world or anywhere on earth but angelic and in heaven.” They also mentioned that Christ would return in glory to judge the living and the dead, repaying all according to their conduct. (Eusebius, Ecclesiastical History, III, xx, 1, 2, 6 [translated by G. A. Williamson])
The letter of Jude and 2 Peter contain quite a number of parallel thoughts. This, however, is to be expected and does not necessarily reflect any direct dependence on the part of either writer on the other. Both Jude and Peter drew on examples found in the Scriptures and other sources, which examples Jews in the first century CE knew well. Peter had close association with James and doubtless also considerable contact with Jude. In view of the same subject matter and apparent interaction between them over a period of years, it should not really be surprising that Peter and Jude used the same well-known examples when addressing matters relating to similar circumstances. Believers were then faced with corruption from inside the congregations. Proponents of error tried to gain followers for themselves, undermined the faith that rested on Christ as the foundation, and diminished the need for disciples of Christ to maintain upright conduct.