In v. 5, Jude reminds the readers of what they already knew about the Israelites: God “…at one time delivered his people out of Egypt, but later destroyed those who did not believe.” This is a significant statement, for through this urgent reminder, Jude recalls the Old Testament principle that people who had been previously “saved” were later destroyed for their unbelief (see also the similar argument in Heb 4). This Old Testament principle is significant in and of itself, but Jude applies it to his New Testament churches. This is the first of several Old Testament lessons in Jude that are retold in order to warn New Testament believers.
The second lesson is similar. Jude points to the fallen angels, explaining that their prior position of security did not keep them from falling into ultimate judgment: “And the angels who did not keep their positions of authority but abandoned their proper dwelling—these he has kept in darkness, bound with everlasting chains for judgment on the great Day.
By “reminding” them of these Old Testament lessons, Jude is issuing warnings to his churches: just because you sit in a favored position does not mean that you cannot forfeit your privileged status. The NLT draws this point out well in its translation of v. 12: “When these people eat with you in your fellowship meals commemorating the Lord’s love, they are like dangerous reefs that can shipwreck you.”
The only clear safeguard against the influence of the false teachers is spiritual growth and abiding in Christ: “But you, dear friends, by building yourselves up in your most holy faith and praying in the Holy Spirit, keep yourselves in God’s love as you wait for the mercy of our Lord Jesus Christ to bring you to eternal life” (vv. 20-21).
Jude concludes with some practical advice to the church about dealing with three different categories of people: “Be merciful to those who doubt; save others by snatching them from the fire; to others show mercy, mixed with fear—hating even the clothing stained by corrupted flesh.” Especially relevant for our topic of security, Jude urges all effort to rescue “from the fire” those members of the congregation who were most in danger.
As happens elsewhere, these apostolic warnings are followed by an encouraging word. The encouraging word does not negate such warnings. Jude does not conclude his letter by saying, “I have issued you warnings, but these warnings are only rhetorical and not to be taken as real threats.” Rather, the word of encouragement is meant to reinforce the teaching that Jesus protects those who abide in him, and this makes for one of the Bible’s most loved benedictions:
To him who is able to keep you from stumbling and to present you before his glorious presence without fault and with great joy— to the only God our Savior be glory, majesty, power and authority, through Jesus Christ our Lord, before all ages, now and forevermore! Amen.
Conclusion: Although it is quite short, the letter of Jude is one such example where verses that contradict eternal security are found in a context which actually does focus on continuance in salvation. By themselves, any prooftexts in Jude are not all that impressive. But when taken together with the larger context of Jude, these verses become all the surer guideposts that warn that believers may shipwreck their faith and fall under the severest judgment.’
source: James M. Leonard, “Eternal Security and Exegetical Overview of the Book of Jude Eternal Security and Making Shipwreck of One’s Faith in the Book of Jude” (arminianbaptist).