John Calvin Commentary 2 Peter 2:22

"It has happened unto them according to the true proverb, The dog turning to his own vomit again, and the sow that had washed to wallowing in the mire." — 2 Peter 2:22 (ASV)

But it has happened unto them. As this example disturbs many, when people who had submitted to the obedience of Christ rush headlong into vices without fear or shame, the Apostle, to remove the offense, says that this happens through their own fault, because they are pigs and dogs. It therefore follows that no part of the sin can be ascribed to the gospel.

For this purpose he quotes two ancient proverbs, the first of which is found as the saying of Solomon in Proverbs 26:11. But what Peter meant is briefly this: the gospel is a medicine that purges us by wholesome vomiting, but there are many dogs who swallow again what they have vomited, to their own ruin. The gospel is also a laver that cleanses all our uncleanness, but there are many swine who, immediately after washing, roll themselves again in the mud. At the same time, the godly are reminded to take heed to themselves, unless they wish to be considered dogs or swine.