To be able always to be laying load on sin is no small degree of mortification. When sin is strong and vigorous, the soul is scarce able to make any head against it; it sighs, and groans, and mourns, and is troubled,as David speaks of himself, but seldom has sin in the pursuit. David complains that his sin had “taken fast hold upon him, that he could not look up,” Ps. 40:12. How little, then, was he able to fight against it!
To know that a man hath such an enemy to deal withal, to take notice of it, to consider it as an enemy indeed, and one that is to be destroyed by all means possible, is required hereunto. As I said before, the contest is vigorous and hazardous,—it is about the things of eternity. When, therefore, men have slight and transient thoughts of their lusts, it is no great sign that they are mortified, or that they are in a way for their mortification. This is every man’s “knowing the plague of his own heart,” 1 Kings 8:38, without which no other work can be done. It is to be feared that very many have little knowledge of the main enemy that they carry about with them in their bosoms. This makes them ready to justify themselves, and to be impatient of reproof or admonition, not knowing that they are in any danger, 2 Chron. 16:10.
– John Owen –
from The Mortification of Sin in Believers, volume 6 of Works, pages 30-31