In Proverbs, chapter twenty-six, verse eleven, “As a dog returneth to his vomit, so a fool returneth to his folly.” The theme is those who once followed Christ, knew the way of righteousness, and then turned back from God and His holy commands to live in sin again.
In the verses from twenty through twenty-two of Second Peter two, in the New Testament, mention this same scripture in greater detail. “For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning. For it had been better for them not to have known the way of righteousness, than, after they have known it, to turn from the holy commandment delivered unto them. 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.”
A dog would rarely return to its own vomit. However, people who throw up from either food poisoning or an upset stomach would not go back to embrace it, but only to clean it up because it gives off an awful scent. This example shows how bad it is for a backslider to go back into something ungodly or a lifestyle thereof, to risk a chance of being completely possessed severely by the powers of darkness. The difference of a former godly person sliding back into the ungodly lifestyle, something they once escaped from, will make it so much more severe to get out of, because those spirits in charge of that territory will ensure that. Similar to someone leaving another and then returning, they will not have it easy.