1:17-23 Sensual men separate from Christ, and his church, and join themselves to the devil, the world, and the flesh, by ungodly and sinful practices. That is infinitely worse than to separate from any branch of the visible church on account of opinions, or modes and circumstances of outward government or worship. Sensual men have not the spirit of holiness, which whoever has not, does not belong to Christ. The grace of faith is most holy, as it works by love, purifies the heart, and overcomes the world, by which it is distinguished from a false and dead faith. Our prayers are most likely to prevail, when we pray in the Holy Ghost, under his guidance and influence, according to the rule of his word, with faith, fervency, and earnestness; this is praying in the Holy Ghost. And a believing expectation of eternal life will arm us against the snares of sin: lively faith in this blessed hope will help us to mortify our lusts. We must watch over one another; faithfully, yet prudently reprove each other, and set a good example to all about us. This must be done with compassion, making a difference between the weak and the wilful. Some we must treat with tenderness. Others save with fear; urging the terrors of the Lord. All endeavours must be joined with decided abhorrence of crimes, and care be taken to avoid whatever led to, or was connected with fellowship with them, in works of darkness, keeping far from what is, or appears to be evil.

18. mockers—In the parallel, 2Pe 3:3, the same Greek is translated, "scoffers." The word is found nowhere else in the New Testament. How Alford can deny that 2Pe 3:2, 3 is referred to (at least in part), I cannot imagine, seeing that Jude quotes the very words of Peter as the words which the apostles used to speak to his (Jude's) readers.

walk after their own ungodly lusts—literally, "after (according to) their own lusts of ungodliness."