Sin doth not only still abide in us, but is still acting, still labouring to bring forth the deeds of the flesh. 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. Sin doth not only abide in us, but “the law of the members is still rebelling against the law of the mind,” Rom. vii. 23; and “the spirit that dwells in us lusteth to envy,” James iv. 5. It is always in continual work; “the flesh lusteth against the Spirit,” Gal. v. 17; lust is still tempting and conceiving sin, James i. 14; in every moral action it is always either inclining to evil, or hindering from that which is good, or disframing the spirit from communion with God. It inclines to evil. “The evil which I would not, that I do,” saith the apostle, Rom. vii. 19. Whence is that? Why, “Because in me (that is, in my flesh) dwelleth no good thing.” And it hinders from good: “The good that I would do, that I do not,” verse 19; — “Upon the same account, either I do it not, or not as I should; all my holy things being defiled by this sin.” “The flesh lusteth against the Spirit, so that ye cannot do the things that ye would,” Gal. v. 17. And it unframes our spirit, and thence is called “The sin that so easily besets us,” Heb. xii. 1; on which account are those grievous complaints that the apostle makes of it, Rom. vii. So that sin is always acting, always conceiving, always seducing and tempting. 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 days. If, then, sin will be always acting, if we be not always mortifying, we are lost creatures. He that stands still and suffers his enemies to double blows upon him without resistance, will undoubtedly be conquered in the issue. If sin be subtle, watchful, strong, and always at work in the business of killing our souls, and we be slothful, negligent, foolish, in proceeding to the ruin thereof, can we expect a comfortable event? There is not a day but sin foils or is foiled, prevails or is prevailed on; and it will be so whilst we live in this world.
John Owen, Of the Mortification of Sin in Believers