1: Jude (Greek for Judas) was Jesus' half brother (Mark 6:3) and described himself as a servant of Jesus. Though earlier in his life Jude rejected Jesus as Messiah, the resurrection is likely the event that changed his mind.
Jude's greeting acknowledges God's sovereignty in our salvation with the words "called" and "kept". We are "called" by the Holy Spirit convicting us of sin, which leads us to repentance and faith in Jesus. We are then "kept" by Jesus' permanent payment for our sins. As fallen beings we are unable to initiate or sustain our salvation.
2. Jude's greeting reflects God's desire that we grow in the Christian values of "mercy, peace, and love".
3-4: He wanted to write about our common salvation in Jesus but it was more urgent to address the apostates who distorted God's grace by using it as an excuse to indulge in sensuality. At the core, their failure to repent of both sensuality and false teaching was a rejection of Jesus. Just as Jude acknowledged God's sovereignty in our salvation, he also acknowledges it in their condemnation. Jude's primary call in this letter is to stand up against the false teachers who want to distort theology and lead others astray.
5-8: Three Old Testament examples of apostasy are given to show how God feels about them and their sins. Jesus (synonymous with God) saved the Israelites from Egypt and everyone 20 years or older perished before seeing the promised land. The rebellious angels were banished from Heaven and await the final judgement when they will be thrown into a lake of fire. And Sodom and Gomorrah were destroyed by fire for perversion. Likewise, the false teachers who reject authority will face judgement as these Old Testament examples did.
8-10: Apostates ignorantly speak blasphemies against the heavenly authorities that God put in place. Even Michael did not judge Satan. To do so would be to put himself in God's place of authority as judge, which would be blasphemy. Instead, he appealed to God's power and authority to rebuke the Devil during a dispute over Moses' body. Little is known about this account since it appears nowhere else in the Bible. It was probably either information Jesus gave to the disciples or directly revealed to Jude by the Holy Spirit.
11: These next few verses are a condemnation of the apostates, describing them with Old Testament examples:
"The way of Cain" was to choose sin over righteousness. After Cain failed to offer from the first fruits of his labor and receive God's approval as Able did, he had a choice to either "do well" or give in to the sinful desires "crouching at his door" and murder Able, which he did. The apostates chose the "way of Cain" by choosing to do evil rather than repent. It is also possible that "the way of Cain" means to reject the system of blood sacrifice for atonement.
Balaam's error was to "gain from wrongdoing" ( 2 Peter 2:15-16, Jude 11). We know from Numbers 22:7 that Balaam was willing to prophesy for money, even if it was for a wicked King who wanted to destroy the people God chose to bless. Though Balaam's error isn't explicitly explained in Numbers, both Peter and Jude link his error with unrighteous personal gain. Therefore, the apostates Jude was condemning must have been profiting financially from their false teaching.
Kora's rebellion was ultimately a rebellion against God (Numbers 16:8-11). Kora and his followers wanted to be priests based on their own holiness rather than acknowledging God's calling of Aaron's descendants as priests (Number's 16:3 and 16:39-40). God's disapproval of their rebellion was clear when He judged them by causing the ground to swallow them up.
12-13: Jude continues to describe these false converts with descriptive metaphors from nature.
"Shepherds feeding themselves": Selfish teachers and leaders who don't care about their flock.
"Waterless clouds and fruitless trees": They promise spiritual nourishment but lack substance.
14-16: Enoch prophesied about the coming judgement against apostates and false teachers for the evil they've done. Verse 16 continues to describe their specific evil deeds.
17-21: 2 Peter 3:3 is a prophecy that these apostates would come at the end of the age and cause trouble in the Church. As we wait for Jesus' mercy (on judgment day) we should not be idle. We must continue the process of sanctification, pray in the Holy Spirit, have mercy on those who doubt and save people by sharing the Gospel with them. "Hating even the garment stained by the flesh" probably means we should be cautious while engaging worldly people. While we love them and share the gospel with them, we should distance ourselves from their sinful lifestyles, remembering our hatred for the sin that separated us from God.
24-25: This great doxology gives Jesus the glory for our salvation and successful battles against sin. Only through Jesus are we presented before the Father as blameless because He graciously paid for our sins and gave us His righteousness.