2 Peter 2:22

‘It has happened to 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.'

What has simply happened is what the old proverb said, they have reverted to nature. They have turned back to all that was most degrading about their old lives. The dog has turned back to his vomit. The thought is perhaps of the scavenger dog which, when it can find no other food, returns to the remains over which it has previously vomited, or even to its own vomit lying in the road, for what nourishment it contains. The returning of such dogs to their own vomit was something that had disgusted men through the ages as Proverbs 26:11 brings out. In the same way the sow, having been washed, has gone back to wallowing in the mud. For both dog and sow seen together elsewhere compare Matthew 7:6. The idea of their degraded and disgusting behaviour was clearly proverbial.