John Calvin on 2 Peter 2:19

19. While they promise them liberty. He shews their inconsistency, that they falsely promised liberty, while they themselves served sin, and were in the worst bondage; for no one can give what he has not. This reason, however, does not seem to be sufficiently valid, because it sometimes happens that wicked men, and wholly unacquainted with Christ, preach usefully concerning the benefits and blessings of Christ. But we must observe, that what is condemned here is vicious doctrine, connected with impurity of life; for the Apostle's design was to obviate the deceptive allurements by which they ensnared the foolish. The name of liberty is sweet, and they abused it for this end, that the hearer, being loosed from the fear of the divine law, might abandon himself unto unbridled licentiousness. But the liberty which Christ has procured for us, and which he offers daily by the gospel, is altogether different, for he has exempted us from the yoke of the law as far as it subjects us to a curse, that he might also deliver us from the dominion of sin, as far as it subjects us to its own lusts. Hence, where lusts reign, and therefore where the flesh rules, there the liberty of Christ has no place whatever. The Apostle then declares this to all the godly, that they might not desire any other liberty but that which leads those, who are set free from sin, to a willing obedience to righteousness.

We hence learn that there have ever been depraved men who made a false pretense to liberty, and that this has been an old cunning trick of Satan. We need not wonder that at this day the same filth is stirred up by fanatical men.

The Papists turn and twist this passage against us, but they thereby betray their ridiculous impudence. For in the first place, men of the filthiest life, in public-houses and brothels, belch out this charge, that we are the servants of corruption, in the life of whom they cannot point out anything reproachful. In the second place, since we teach nothing respecting Christian liberty but what is derived from Christ and his Apostles, and at the same time require the mortification of the flesh, and the proper exercises for subduing it, much more strictly than they do who slander us, they vomit forth their curses, not so much against us as against the Son of God, whom we have as our certain teacher and authority.

For of whom a man is overcome. This sentence is derived from military law; but yet it is a common saying among heathen writers, that there is no harder or a more miserable bondage than when lusts rule and reign. What then ought to be done by us, on whom the Son of God has bestowed his Spirit, not only that we may be freed from the dominion of sin, but that we may also become the conquerors of the flesh and the world?

2 Peter 2:20-22

20. 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.

20. Nam si ii qui sufugerant ab inquinamentis mundi per cognitionem Domini et Servatoris Jesu Christi, rursum iisdem impliciti superantur, facta sunt illis postrema pejora prioribus.

21. 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.

21. Melius enim ipsis esset non cognovisse viam justitiae, qum ubi cognoverunt converti ab eo, quod illis traditum fuit, sancto praecepto.

22. 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.

22. Sed accidit illis quod vero proverbio dicitur, Canis reversus ad proprium vomitum; et sus lota, ad volutabrum coeni.