This is a small exposition by the greatest Puritain theologian, John Owen, on Romans 8:13.

For if you live according to the flesh you will die, but if by the Spirit you put to death the deeds of the body, you will live. “Put to death” = mortify. “the deeds of the body” = sin. That’s the point of this book, and so far it has been incredible. I’ll put down various quotes I’ve been gleaning from it in the meantime until I finish it. Mortification from a self-strength, carried on by ways of self-invention, unto the end of a self-righteousness, is the soul and substance of all false religion in the world. The mortification of indwelling sin remaining in our mortal bodies, that it may not have life and power to bring forth the works or deeds of the flesh is the constant duty of believers. Indwelling sin is compared to a person, a living person, called “the old man,” with his faculties, and properties, his wisdom, craft, subtlety, strength; this, says the apostle, must be killed, put to death, mortified, — that is, have its power, life, vigour, and strength, to produce its effects, taken away by the Spirit. It is, indeed, meritoriously, and by way of example, utterly mortified and slain by the cross of Christ; and the “old man” is thence said to be “crucified with Christ.” Do you mortify; do you make it your daily work; be always at it whilst you live; cease not a day from this work; be killing sin or it will be killing you. Your being dead with Christ virtually, your being quickened with him, will not excuse you from this work. When sin lets us alone we may let sin alone; but as sin is never less quiet than when it seems to be most quiet, and its waters are for the most part deep when they are still, so ought our contrivances against it to be vigorous at all times and in all conditions, even where there is least suspicion. Who can say that he had ever any thing to do with God or for God, that indwelling sin had not a hand in the corrupting of what he did? And this trade will it drive more or less all our day. The saints, whose souls breathe after deliverance from [indwelling sin's] perplexing rebellion, know there is no safety against it but in a constant warfare. This is the saddest warfare that any poor creature can be engaged in. A soul under the power of conviction from the law is pressed to fight against sin, but hath no strength for the combat. They cannot but fight, and they can never conquer; they are like men thrust on the sword of enemies on purpose to be slain. The law drives them on, and sin beats them back. Sometimes they think, indeed, that they have foiled sin, when they have only raised a dust that they see it not; that is, they distemper their natural affections of fear, sorrow, and anguish, which makes them believe that sin is conquered when it is not touched. By that time they are cold, they must to the battle again; and the lust which they thought to be slain appears to have had no wound. Now, though doubtless there may, by the Spirit and grace of Christ, a wonderful success and eminency of victory against any sin be attained, so that a man may have almost constant triumph over it, yet an utter killing and destruction of it, that it should not be, is not in this life to be expected. When a man on some outward respects forsakes the practice of any sin, men perhaps may look on him as a changed man. God knows that to his former iniquity he hath added cursed hypocrisy, and is got in a safer path to hell than he was in before. He hath got another heart than he had, that is more cunning; not a new heart, that is more holy. Men in age do not usually persist in the pursuit of youthful lusts, although they have never mortified any one of them. And the same is the case of bartering of lusts, and leaving to serve one that a man may serve another. He that changes pride for worldliness, sensuality for Pharisaism, vanity in himself to the contempt of others, let him not think that he hath mortified the sin that he seems to have left. He hath changed his master, but is a servant still. To be quit of this, men resolve at such times against their sins. Sin shall never more have any place in them; they will never again give up themselves to the service of it. Accordingly, sin is quiet, stirs not, seems to be mortified; not, indeed, that it hath received any one wound, but merely because the soul hath possessed its faculties, whereby it should exert itself, with thoughts inconsistent with the motions thereof; which, when they are laid aside, sin returns again to its former life and vigour.