For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live. (Rom 8:13 KJV)
…we say to a sick man, “If you will take such a potion, or use such a remedy, you will be well.” The thing we solely intend to express is the certainty of the connection that is between the potion or remedy and health. And this is the use of it here. The certain connection that is between the mortifying of the deeds of the body and living is intimated in this conditional particle.
Now, the connection and coherence of things being manifold, as of cause and effect, of way and means and the end, this between mortification and life is not of cause and effect properly and strictly, —for “eternal life is the gift of God through Jesus Christ,” Rom. vi.23,—but of means and end. God hath appointed this means for the attaining that end, which he hath freely promised. Means, though necessary, have a fair subordination to an end of free promise.
– John Owen –
from The Mortification of Sin in Believers; volume 6 of Works, page 6