HIS book is devoted almost entirely to the description of apostates. Remember, an apostate is someone who, knowing the gospel & having followed from afar, is yet guilty of rejecting Christ (vs. 4). An apostate is an unbeliever. Yet not all unbelievers are apostates. Hebrews indicates that apostates are in a hopeless condition (Hebrews 6:4 - 6 & 10:26 - 27). Certainly, there is hope for lost souls in general (which is indicated here in vs. 22 - 23). Still, in our fellowship with God & in our service to Him, there are battles that must be fought. In other words, as we work to bring sheep into the fold (evangelism), there are wolves that we must be willing to ward off. Jude calls this "contending for the faith" (vs. 3).
Jude defines “saints” clearly. We are the ones who are sanctified & preserved (vs. 1). We hold to a "common salvation" - a single faith (vs. 3). We are the recipients of God's grace & the acceptors of the Lord Christ (vs. 4). We have the Spirit, love, & mercy of God; & eternal life (vs. 19 - 21). We are emissaries of mercy; evangelists bearing the gospel (vs. 22 - 23). To put it simply, we have been saved from our sin (Jude 1:23 - 24). The ungodly apostates (who Jude would have us fight against) have none of these things.
An apostate turns from the truth. Jude gives many examples of apostates (& of apostasy in general). Think of the Jews who came out of Egypt, (vs. 5) & the angels who fell with Lucifer (vs. 6). How about the sodomites (vs. 7 & 10). Cain, Balaam & Korah are also examples of men who were guilty of apostasy (Jude 1:11). In every case we can say categorically that they knew better and had opportunities to commit themselves to the truth. EACH ONE chose error instead.