“The choicest believers, who are assuredly freed from the condemning power of sin, ought yet to make it their business all their days to mortify the indwelling power of sin.
The principal efficient cause of the performance of this duty is the Spirit: “If by the Spirit.” The Spirit here is the Spirit mentioned in verse 11, the Spirit of Christ, the Spirit of God, that “dwells in us,” verse 9, that “quickens us,” verse 11; “the Holy Ghost,” verse 14; the “Spirit of adoption,” verse 15; the Spirit “that maketh intercession for us,” verse 26. All other ways of mortification are in vain, all helps leave us helpless; it must be done by the Spirit. Men, as the apostle intimates, Rom. ix. 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, saith 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.” – John Owen, The Mortification of Sin in Believers, Vol. 6, pg. 7
Commenting on Romans 8:13