John Owen on fighting an enemy that’s hidden deep within:
This then is the dwelling place of sin– the human heart. Here dwells our enemy. Within this fort the tyrant sin maintains its rebellion against God all our days. Like an enemy at war, it is not just his numbers and force of men under arms that are to be feared, but also the impregnable fortress that he possesses. Such is the heart to this enemy of God and of our souls. Let us then examine some of the features of this fortress.
In the first place the heart is unsearchable. “The heart is deceitful above all things, and desperately wicked: who can know it? I the LORD search the heart…” (Jer. 17:9-10). The heart of man is known only to God. We do not know the hearts of others. We do not even know the secret intrigues and schemes, twists and turns, actions and tendencies of our own hearts. All but the infinite, all-seeing God are utterly ignorant of these things.
In the unsearchable heart resides the law of indwelling sin. Herein lies much of its security and strength. We fight an enemy whose strength is secret and whose presence is hidden. So often when we think sin has been destroyed, it is merely out of sight. It has coverts and retreats into which we cannot pursue.
Sin lies so camouflaged– in the darkness of the mind, in the indisposition of the will, and in the worldliness of the affections– that no eye can discover it. The best of our wisdom is but to look out for its first appearances. But since nothing lies in the unsearchable heart that is not “naked and open” to God (Heb. 4:13), it is the Lord alone who penetrates to the roots of things.