See here, 1. What an abominable thing sin is, and how hateful sometimes it is made to appear, even to the sinner himself. When his conscience is convinced, or he feels smart from his sin, he is sick of it, and vomits it up; he seems then to detest it and to be willing to part with it.
It is in itself, and, first or last, will be to the sinner, more loathsome than the vomit of a dog, Psalm 36:2. 2. How apt sinners are to relapse into it notwithstanding. As the dog, after he has gained ease by vomiting that which burdened his stomach, yet goes and licks it up again, so sinners, who have been convinced only and not converted, return to sin again, forgetting how sick it made them.
The apostle (2 Peter 2:22) applies this proverb to those that have known the way of righteousness but are turned from it; but God will spue them out of his mouth, Revelation 3:16.