4 certain people . . . crept in unnoticed. The troublemakers apparently have come from outside the particular church Jude addresses, perhaps posing as itinerant prophets or teachers (2 John 10, 11).

designated for this condemnation. This difficult phrase probably refers to various prophecies about the coming and condemnation of ungodly men like the false teachers, perhaps including the prophecy of Enoch in vv. 14, 15 and the apostolic prophecies in vv. 17, 18. Or less likely, it may refer to the fate of the wicked being written in heavenly books (Jer. 22:30; Rev. 17:8).

pervert the grace of our God into sensuality. Jude’s opponents were guilty of antinomianism—the belief that Christians are under no obligation to follow the moral law as a rule of life. Such teaching was a persistent problem in the early church (Rom. 3:8; 6:15; 1 Cor. 6:12–15; Gal. 5:13), especially where Paul’s emphasis upon justification by grace through faith was misunderstood and perverted.

deny . . . Master . . . Lord, Jesus Christ. By their godless and immoral behavior the false teachers deny Christ. The designation of Christ as “Lord” recognizes Christ’s deity.