John Calvin Commentary 2 Peter 2:20

"For if, after they have escaped the defilements of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein and overcome, the last state is become worse with them than the first." — 2 Peter 2:20 (ASV)

For if after. He again shows how destructive was the sect which led men consecrated to God back to their old filth and the corruptions of the world. And he shows the severity of the evil by a comparison, for it was no common sin to depart from the holy doctrine of God. It would have been better for them, he says, not to have known the way of righteousness; for though there is no excuse for ignorance, yet the servant who knowingly and willfully despises the commands of his lord deserves a twofold punishment. Moreover, there was ingratitude, because they willfully extinguished the light of God, rejected the favor conferred on them, and having shaken off the yoke, became perversely rebellious against God; indeed, as far as they could, they profaned and abolished the inviolable covenant of God, which had been ratified by the blood of Christ.

The more diligent, then, we ought to be to advance humbly and carefully in the course of our calling. We must now consider each sentence.

By naming the pollutions of the world, he shows that we wallow in filth and are wholly polluted until we renounce the world. By the knowledge of Christ, he undoubtedly means the gospel. He testifies that its purpose is to deliver us from the defilements of the world and to lead us far away from them. For the same reason, he afterwards calls it the way of righteousness. He then alone makes true progress in the gospel who faithfully learns Christ; and he truly knows Christ who has been taught by Him to put off the old man and to put on the new man, as Paul reminds us in Ephesians 4:22.