“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 villainy: 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— 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.”
That is the “enemy within.”
But exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin” (Hebrews 3:13).
―John Owen, The Mortification of Sin