Reading "The Mortification of Sin" by John Owen has given me some insight into the nature of sin.
First, sin is active. It is not a passive thing. It is something that grows and expands. Owen gives this as a reason why we must fight it so vigorously.
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.
It is naive to think that temptations are by chance. Sin strikes at the opportune moment. Surely many of us regard sin as an enemy, but do we underestimate the cleverness of its attacks?
Sin is also deceitful, and not in an obvious way. Those who do not notice the deceit have been deceived, and that has happened to me many times! Sin will ask for something so small and so seemingly innocent at first.
...herein lies no small share of the deceitfulness of sin, by which it prevails to the hardening of men, and so to their ruin, Hebrews 3:13, -- it is modest, as it were, in its firsts motions and proposals, but having once got footing in the heart by them, it constantly makes good its ground, and presently on to some farther degrees in the same kind.
If I used to think of sin as a man who I was openly fighting, now I think of it as a pretended friend who is always betraying and plotting against me. How much more dangerous! How much harder to defend against!
Sin will also pretend to be dead when we set out to mortify it. It is crafty and it seeks to preserve itself by any means necessary.
...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 vigor.
If we are to mortify sin, we must look to the Spirit for guidance and power, for sin has far more energy and guile than we could ever conquer on our own, and we must remember what the Spirit teaches us, or else we will fall into the same deceptions!