Although Jude was very eager to write to his readers about salvation, he felt that he must instead warn them about certain immoral men circulating among them who were perverting the grace of God. Apparently these false teachers were trying to convince believers that being saved by grace gave them license to sin since their sins would no longer be held against them.
This epistle is addressed to all believers in the gospel. Its design appears to be to guard believers against the false teachers who had begun to creep into the Christian church, and to scatter dangerous tenets, by attempting to lower all Christianity into a merely nominal belief and outward profession of the gospel. Having thus denied the obligations of personal holiness, they taught their disciples to live in sinful courses, at the same time flattering them with the hope of eternal life. The vile character of these seducers is shown, and their sentence is denounced, and the epistle concludes with warnings, admonitions, and counsels to believers.Matthew Henry