The need for Jude to “switch topics” and admonish Christians to stand up for the faith (instead of celebrating the joy of salvation, vs. 3) was rooted in the fact that “certain people” (vs. 4) were trying to change and corrupt the body of Christ. These were people who shamelessly masqueraded as faithful Christians in order to promote wanton behavior (“lasciviousness,” KJV) in place of the morality God’s grace would produce. (It seems they were using “grace” as an excuse to participate in and defend sin.) Jude says that by doing this, they were actually “cancelling” (nullifying, negating) God and Christ, because what they did and taught produced the opposite of the moral restraint the gospel teaches.
The fact that Jude says these perverse people were “ordained from of old” is not a declaration that they had no choice in the matter, or that God “created” them to be lost. What Jude is emphasizing is that God’s basic standard and concept of morality has never changed over the course of time. Just as unbelieving Israelites, disobedient angels, and the perversions of Sodom and Gomorrah led to condemnation (vv. 5-7), so would the perversions promoted by these individuals. Those examples are stand as incontrovertible evidence of what it costs to put sinful desires above God’s commandments.
Jude appeals to examples of saved people who fell along with examples of people who were judged by God to deliver a basic warning, and an admonition. The warning, in vv. 8, 10, 11, & 16, is simple: “Don’t follow their example OR their teaching.” The admonition tells these Christians to ‘innoculate’ themselves with the love of God, vv. 20-21. The fact that Jude mentions both the archangel Michael (vs. 9) and describes Enoch as a prophet (vs. 14) supports the idea that the original readers of this brief letter were probably Jews who had become Christians. They would likely identify the name Michael with the “chief prince” mentioned in Daniel chapters 10 & 12, and the description of Enoch as both the seventh generation from Adam AND a prophet would cement in Jewish minds the man who both fathered Methuselah (Genesis 5:18-23) and who never died because God “took” him (Genesis 5:24). Both of these are presented as examples of fidelity to God and His word that should be familiar to the (original) readers, and who did not try to “go beyond” the limits of His word. The “lesson” the readers should take away comes in vv. 17-19 – they should not be “surprised” or disillusioned in their faith by the fact that some who claimed to be Christians would turn out to be deceitful and worldly. After all, the apostles had foretold exactly such behavior (cf. 2 Peter 3:2-4), just as Jesus had forewarned them that one of their own number would betray Him (Matthew 26:21-25).
The ultimate application of Jude’s warning comes in vv. 20-24, as Jude first counsels the saints to pay attention to their own spiritual condition, and then urges them to act whenever they saw a brother or sister falling into danger through temptation or sin. Note the distinction he makes in “how” Christians should deal with this situation, in the different responses of vv. 22-23 – some require tender nurturing, to encourage and strengthen them (the “compassion” of vs. 22), but others require a far stronger, more “blunt” response to shock them into sensibility regarding their danger (vs. 23). In vs. 24 Jude reminds us yet again that it is God’s assistance (the strength we gain by consistently relying on HIM) that is able to keep us from falling into such sins.