John Calvin Commentary 2 Peter 2:21

"For it were better for them not to have known the way of righteousness, than, after knowing it, to turn back from the holy commandment delivered unto them." — 2 Peter 2:21 (ASV)

By saying that having forsaken the commandment delivered unto them, they returned to their own pollutions, he indicates 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 specifically taught to those whom God favors with the light of his gospel. But he declares that those who make themselves slaves again to the pollutions of the world fall away from the gospel. The faithful also do indeed sin; but as they do not allow sin dominion, they do not fall away from the grace of God, nor do they renounce the profession of sound doctrine which they have once embraced. For they are not to be considered conquered, while they strenuously resist the flesh and its lusts.