"Sin does not only still abide in us, but is still acting, still laboring 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."
"Who can say that he had ever anything 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 while we live in this world."
"This is one main reason why the Spirit and the new nature are given unto us--that we may have a principle within us whereby to oppose sin and lust. 'The flesh lusts against the Spirit.' Well! and what then? Why, 'the Spirit also lusts against the flesh' (Gal. 5:17). There is a propensity in the Spirit, or spiritual new nature, to be acting against the flesh, as well as in the flesh to be acting against the Spirit (2 Pet. 1:4-5). It is our participation of the divine nature that gives us an escape from the pollutions that are in the world through lust; and there is a law of the mind (Rom. 7:23), as well as a law of the members. Now this is, first, the most unjust and unreasonable thing in the world, when two combatants are engaged, to bind one and keep him up from doing his utmost and to leave the other at liberty to wound him at his pleasure; and, secondly, the most foolish thing in the world to bind him who fights for our eternal condition and to let him alone who seeks and violently attempts our everlasting ruin. The contest is for our lives and souls. Not to be daily employing the Spirit and new nature for the mortifying of sin is to neglect that excellent succor which God has given us against our greatest enemy. If we neglect to make use of what we have received, God may justly hold his hand from giving us more. His graces, as well as his gifts, are bestowed on us to use, exercise, and trade with. Not to be daily mortifying sin is to sin against the goodness, kindness, wisdom, grace, and love of God, who has furnished us with a principle of doing it."
These passages are from Overcoming Sin and Temptation: Three Classic Works by John Owen, ed. by Kelly M. Kapic and Justin Taylor (Wheaton, IL: Crossway Books, 2006), pp. 51, 52, 53-54. I highly recommend you get ahold of this book and read it (I just started it this past weekend).