But it is happened unto them according to the true proverb, The dog is turned to his own vomit again; and the sow that was washed to her wallowing in the mire. According to the true proverb - This seems to be a reference to Proverbs 26:11 : ככלב שב אל קאו kekeleb shab al keo; as the dog returneth to his vomit, so a fool repeateth his folly. In substance this proverb is found among the rabbins; so Midrash Ruth, in Sohar Chadash, fol. 62: Orphah is returned to her mire, Ruth persevered in spirit; and again, Ibid. fol. 64: "Orphah, which is נפש הבהמית nephesh habbehemith, the bestial soul, is returned to her mire."
The Greeks have something like it; so Arrian, Dissert. Epict. l. iv. c. 11, says: Απελθε και χοιρῳ διαλεγου, ἱν' εν βορβορῳ μη κυλιηται, "Go and reason with the swine, lest he be rolled in the mire." This is called a true proverb: for it is a fact that a dog will eat up his own vomit; and the swine, howsoever carefully washed, will again wallow in the mire. As applied here it is very expressive: the poor sinner, having heard the Gospel of Christ, was led to loathe and reject his sin; and, on his application to God for mercy, was washed from his unrighteousness. But he is here represented as taking up again what he had before rejected, and defiling himself in that from which he had been cleansed.
Here is a sad proof of the possibility of falling from grace, and from very high degrees of it too. These had escaped from the contagion that was in the world; they had had true repentance, and cast up "their soursweet morsel of sin;" they had been washed from all their filthiness, and this must have been through the blood of the Lamb; yet, after all, they went back, got entangled with their old sins, swallowed down their formerly rejected lusts, and rewallowed in the mire of corruption. It is no wonder that God should say, the latter end is worse with them than the beginning: reason and nature say it must be so; and Divine justice says it ought to be so; and the person himself must confess that it is right that it should be so. But how dreadful is this state! How dangerous when the person has abandoned himself to his old sins! Yet it is not said that it is impossible for him to return to his Maker; though his case be deplorable, it is not utterly hopeless; the leper may yet be made clean, and the dead may be raised. Reader, is thy backsliding a grief and burden to thee? Then thou art not far from the kingdom of God; believe on the Lord Jesus, and thou shalt be saved.
Commentary on the Bible, by Adam Clarke [1831].