"As a dog returneth to his vomit, so a fool returneth to his folly" (Pro 26:11 KJV).
When an Old Testament verse in quoted in the New Testament, look for its primary meaning to be in the former passage. In this case, Peter writes: "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" (2Pe 2:21-22 KJV).
The salvation by works crowd love verses like this because it shows, in their minds, that a saved man can return to his former state and end up in hell because he didn't do his part to be saved.
We have been looking a lot at the fool in Proverbs, and the one thing that we know is that the fool is not interested in knowing God, “How long, ye simple ones, will ye love simplicity? and the scorners delight in their scorning, and fools hate knowledge?” (Pro 1:22 KJV).
The fool isn't interested in heaven. You speak of heavenly things and they'll say, "I'll worry about such things later." They only want to live for the present, and to have their fun. So, a fool will experiment with life. He'll try religion for a bit, so long as it is fun, but as soon as the preaching gets too hot--he'll return to the things that were more comfortable.
There is a huge difference between "knowing the way of righteousness," that is, knowing that salvation is in Christ, but not following it to it's conclusion of receiving Christ. These people have a head knowledge, but they have never had a heart knowledge--they are missing salvation by 18 inches.
The believer has embraced Him Who is the very embodiment, the personification of "wisdom" and have a different kind of knowledge: "He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son. And this is the record, that God hath given to us eternal life, and this life is in his Son. He that hath the Son hath life; and he that hath not the Son of God hath not life. These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God" (1Jn 5:10-13 KJV).
Lord, we thank you for the gift of eternal life that You have given to us through the Cross and for the assurance that You are with us, even unto the very end of time. Amen.