Unless a man be a believer,—that is, one that is truly ingrafted unto Christ,—he can never mortify any one sin; I do not say, unless he know himself to be so, but unless indeed he be so.
Mortification is the work of believers: Rom. 8:13, “If ye through the Spirit,” etc.,—ye believers, to whom there is no condemnation, verse 1. They alone are exhorted to it: Col. 3:5, “Mortify therefore your members which are upon the earth.” Who should mortify? You who “are risen with Christ,” verse 1; whose “life is hid with Christ in God,” verse 3; who “shall appear with him in glory,” verse 4. An unregenerate man may do something like it; but the work itself, so as it may be acceptable with God, he can never perform. You know what a picture of it is drawn in some of the philosophers,—Seneca, Tully, Epictetus; what affectionate discourses they have of contempt of the world and self, of regulating and conquering all exorbitant affections and passions! The lives of most of them manifested that their maxims differed as much from true mortification as the sun painted on a sign-post from the sun in the firmament; they had neither light nor heat. Their own Lucian sufficiently manifests what they all were. There is no death of sin without the death of Christ. You know what attempts there are made after it by the Papists, in their vows, penances, and satisfactions. I dare say of them (I mean as many of them as act upon the principles of their church, as they call it) what Paul says of Israel in point of righteousness, Rom. 9:31, 32,—They have followed after mortification, but they have not attained to it. Wherefore? “Because they seek it not by faith, but as it were by the works of the law.” The same is the state and condition of all amongst ourselves who, in obedience to their convictions and awakened consciences, do attempt a relinquishment of sin;—they follow after it, but they do not attain it.

Owen, J. (n.d.). The works of John Owen. (W. H. Goold, Ed.) (Vol. 6, pp. 33–34). Edinburgh: T&T Clark.