What follows below is an attempt to piece together the life of Jude as told by the Bible, using what few references to him that we have.
Jude grew up with Jesus and his other siblings in the house of Joseph and Mary and thus enjoyed being from the line of David. Being last in the list of four brothers (Matt 13:55), he may have been the youngest of them all, with sisters (at least two) scattered somewhere in the lineup.
In the book that bears his name, assuming this Jude was indeed its author, we see that he identified himself as the “brother of James” (Jude 1), who, as this was a notable James, must have been the leader of the early Jerusalem church (cf. Acts 12:17; 15:13; 21:18).
Both James and Jude (and Joses and Simon) were the biological half-brothers (or more simply, “brothers”) of Jesus, the most natural reading of the text (Matt 13:55). They were neither the cousins of Jesus as Roman Catholicism believes nor stepbrothers from Joseph’s supposed previous marriage as Eastern Orthodoxy believes, both theories stemming from the erroneous notion of the perpetual virginity of Mary.
Jude likely learned to be a carpenter like his father Joseph. Maybe he was old enough to notice when Joseph and Mary were looking for the twelve-year-old Jesus (Luke 2:41–52). Maybe not. One way or the other, he would have eventually noticed something unique about Jesus as he grew up. His older Brother never sinned!
From how his hometown reacted to Jesus in Matthew 13:53–58 (cf. Mark 6:1–6; Luke 4:16–30), and thus not in the same way to Jude or any of his brothers, we could surmise that Jude did not have unusual wisdom or the ability to do mighty works, astonished no one with any teaching, and therefore offended no one. He was not a prophet and tried to afford what honor his hometown would give him.
In fact, there are multiple occasions which showed his unbelief toward Jesus. First, while Jesus was preaching in His home and unable to eat because of the crowd, his mother and brothers stood outside, trying to seize Him, claiming, “He is out of His mind!” (Mark 3:20–21; cf. Matt 12:46–; Luke 8:19–21). Second, not long thereafter, they were seeking Jesus again, and Jesus responded that His mother and brothers were present in the crowd before Him (Mark 3:31–35), that is, that those who were listening to Him and following His words were His spiritual family and took priority over His physical family, a rebuke in the ears of His physical family indeed. Third, His brothers misunderstood that He should suffer and urged Him to do miracles in Jerusalem in order to show Himself to the world, becoming the King right away (John 7:1–4). In spite of all the miracles of Jesus they had witnessed or heard about (cf. Matt 13:58; John 2:12), “not even His brothers believed in Him” (John 7:5).
This unbelief would not last, however. Perhaps Mary told her family that she had seen the risen Jesus (cf. Matt 28:1–10). Perhaps James told his family about when he saw the risen Jesus as well (1 Cor 15:7). Perhaps these testimonies sat heavily upon Jude along with the fact that so many others had already believed in Him and had been doing so for multiple years. Whenever Jude’s conversion was, it was probably sometime between the resurrection and Pentecost. He was not with his mother at the cross (cf. John 19:25), but we do find Him praying with Mary and his brothers in Acts 1:14.
In addition to being saved, we also see him serving. He may have been one Lord’s brothers who traveled with his wife for missionary work in 1 Cor 9:5, something Paul wrote in AD 55. One or two decades later, Jude wrote the letter that bears his name. Far from unbelief, he called himself a slave of his half-brother Jesus who he identified as Lord, Master, and Christ (Jude 1, 4). He had an eagerness to speak of salvation in Him (Jude 3) and readily warned the church about false teachers (Jude 4–16). He followed the apostles on this matter (Jude 17–19). He saw it necessary to keep one’s self in the love of God and that God would likewise keep him from stumbling until the day he was presented as perfect and blameless before Him—all through Jesus Christ (Jude 24–25). As a result, Jude could do no less than leave us with one of the greatest doxologies in Scripture in Jude 24–25, showing us not only evidence of his conversion, but of someone absolutely captivated by the glory of God, an example for us all today:
“Now to him who is able to keep you from stumbling and to present you blameless before the presence of his glory with great joy, to the only God, our Savior, through Jesus Christ our Lord, be glory, majesty, dominion, and authority, before all time and now and forever. Amen.”
All quotes ESV