It is true, it is, it will be, required of every person whatever that hears the law or gospel preached, that he mortify sin. It is his duty, but it is not his immediate duty; it is his duty to do it, but to do it in God’s way. If you require your servant to pay so much money for you in such a place, but first to go and take it up in another, it is his duty to pay the money appointed, and you will blame him if he do it not; yet it was not his immediate duty,—he was first to take it up, according to your direction. So it is in this case: sin is to be mortified, but something is to be done in the first place to enable us thereunto.
…it is the Spirit alone that can mortify sin; he is promised to do it, and all other means without him are empty and vain. How shall he, then, mortify sin that hath not the Spirit? A man may easier see without eyes, speak without a tongue, than truly mortify one sin without the Spirit.
– John Owen –
from The Mortification of Sin in Believers, volume 6 of Works, page 34