For if you live according to the flesh you will die, but if by the Spirit you put to death the deeds of the body, you will live. – Romans 8:13
The principal efficient cause of the performance of this duty is the Spirit: ei de pneumati—“if by the Spirit.” The Spirit here is the Spirit mentioned [in Rom. 8] verse 11, the Spirit of Christ, the Spirit of God, that “dwells in us” (v. 9), that “quickens us” (v. 11); “the Holy Ghost” (v. 14); the “Spirit of adoption” (v. 15); the Spirit “that makes intercession for us” (v. 26). All other ways of mortification are vain, all helps leave us helpless; it must be done by the Spirit. Men, as the apostle intimates (Rom. 9:30-32), may attempt this work on other principles, by means and advantages administered on other accounts, as they always have done, and do; but, says he, “This is the work of the Spirit; by him alone is it to be wrought, and by no other power is it to be brought about.” Mortification from a self-strength, carried on by ways of self-invention, unto the end of a self-righteousness, is the soul and substance of all false religion in the world.
Overcoming Sin & Temptation (Wheaton, IL; Crossway; 2006) p. 47.