It would have been better for them not to have known the way of righteousness than to have known it and then to turn away from the holy commandment passed on to them.
21. By saying that having forsaken the commandment delivered unto them, they returned to their own pollutions, he intimates first, how inexcusable they were; and secondly, he reminds us that the doctrine of a holy and virtuous life, though common to all and indiscriminately belonging to all, is yet peculiarly taught to those whom God favors with the light of his gospel.
The apostle's design being to warn us of, and arm us against, seducers, he now returns to discourse more particularly of them, and give us an account of their character and conduct, which abundantly justifies the righteous Judge of the world in reserving them in an especial manner for the most severe and heavy doom, as Cain is taken under special protection that he might be kept for uncommon vengeance.
Commenting on 2 Peter 2:10-22
For it had been better for them,.... Not that ignorance is good, or to be excused; but it would have been a lesser evil, and not so much aggravated: not to have known the way of righteousness; the same with "the way of truth", Pe2 2:2, and "the right way", Pe2 2:15, the Gospel, which points out the way and method of a sinner's justification...