Sin will not only be striving, acting, rebelling, troubling, disquieting, but if let alone, if not continually mortified, it will bring forth great, cursed, scandalous, soul-destroying sins. The apostle tells us what the works and fruits of it are, Gal. v. 19–21, “The works of the flesh are manifest, which are, adultery, fornication, uncleanness, lasciviousness, idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, envyings, murders, drunkenness, revellings, and such like.” You know what it did in David and sundry others. Sin aims always at the utmost; every time it rises up to tempt or entice, might it have its own course, it would go out to the utmost sin in that kind. Every unclean thought or glance would be adultery if it could; every covetous desire would be oppression, every thought of unbelief would be atheism, might it grow to its head. Men may come to that, that sin may not be heard speaking a scandalous word in their hearts, — that is, provoking to any great sin with scandal in its mouth; but yet every rise of lust, might it have its course, would come to the height of villany: it is like the grave, that is never satisfied. And herein lies no small share of the deceitfulness of sin, by which it prevails to the hardening of men, and so to their ruin, Heb. iii. 13, — it is modest, as it were, in its first motions and proposals, but having once got footing in the heart by them, it constantly makes good its ground, and presseth on to some farther degrees in the same kind. This new acting and pressing forward makes the soul take little notice of what an entrance to a falling off from God is already made; it thinks all is indifferent well if there be no farther progress; and so far as the soul is made insensible of any sin, — that is, as to such a sense as the gospel requireth, — so far it is hardened: but sin is still pressing forward, and that because it hath no bounds but utter relinquishment of God and opposition to him; that it proceeds towards its height by degrees, making good the ground it hath got by hardness, is not from its nature, but its deceitfulness. Now nothing can prevent this but mortification; that withers the root and strikes at the head of sin every hour, so that whatever it aims at it is crossed in. There is not the best saint in the world but, if he should give over this duty, would fall into as many cursed sins as ever any did of his kind.
[src: Of the Mortification of Sin in Believers, Chapter 2, heading 3]