John Owen uses the analogy of a war when speaking of mortifying sin. Although this may not be theologically exact, this is how I have been thinking about it.
Before we believe in Christ, our hearts is like a country that has fallen to an enemy. The enemy has control over the land. Civilians flee and go into hiding. Injustice and corruption is everywhere. Our army is scattered and powerless. This is a heart under sin.
When we believe in Christ and he sends us his Spirit, instantly the country belongs to him. The death toll has sounded for the enemy. At some point they will be overthrown, that much is certain. But when?
Though we have been washed pure by the blood of Jesus, we still have habits of sin that plague us. The enemies that are in the country do not leave immediately, nor will they leave without a fight.
Christ gives us his Spirit, the Spirit of victory and hope. Our army, though small and hiding in some secret wood from the enemy, now has a captain to rally around. Following him, we can be confident in victory.
Something that still remains is for us to listen to the Spirit, to keep in step with the Spirit, to follow the Spirit into battle. Perhaps the Spirit would better be imagined as a counselor who knows the ways of war, and how victory will be won, and Christ as the captain who we follow into battle.
My revelation was that we are still in the wood outside of some city. The city, though it rightfully belongs to Christ, has not yet been conquered. Though he fights with us, and he is the one to whom the victory belongs, still there is fighting to be done by us.
That fighting, to take back the capitol city from the enemy, to win skirmishes here and there in the countryside, to withstand large assaults and sieges on other cities, to quell fights in the outer reaches of the country, to rebuild the defenses of the city, to cultivate culture once more, this is the mortification of sin.
To leave a sin unmortified is to leave the country in control of the enemy and to be content to be a small band of rebels hiding in the forest. To be sure, someday Jesus himself will conquer all and bring all under the his authority, to which it already belonged the moment we were saved.
But what might he say to us, if when he returns he finds the country still largely under control of the enemy? I think he would ask, why did you not claim it? Why did you not join with me to take back what is mine? Why did you not listen to the counselor I gave you, who taught you how victory could be won?
Here is the passage in The Mortification of Sin that got me thinking about this analogy.
To labor to be acquainted with the way, wiles, methods, advantages, and occasions of its [sin] success, is the beginning of this warfare. So do men deal with enemies. The inquire out their counsels and designs, ponder their ends, consider how and by what means they have formerly prevailed, that they may be prevented...
So do they deal with lust who mortify it indeed. Not only when it is actually vexing, enticing, seducing, but in their retirements they consider, "This is our enemy; this is his way and progress, these are his advantages, thus hath he prevailed, and thus he will do, if not prevented...
And, indeed, one of the choicest and most eminent parts of practically spiritual wisdom consists in finding out the subtleties, policies, and depths of any indwelling sin; to consider and know wherein its greatest strength lies, -- what advantages it uses to make of its occasions, opportunities, temptations, -- what are its pleas, pretenses, reasonings, -- what its stratagems, colors, excuses; to set the wisdom of the Spirit against the craft of the old man...