John Calvin Commentary Romans 8:13

"for if ye live after the flesh, ye must die; but if by the Spirit ye put to death the deeds of the body, ye shall live." — Romans 8:13 (ASV)

For if you will live after the flesh, etc. He adds a threatening to more effectively shake off their lethargy. By this, those who boast of justification by faith without the Spirit of Christ are also fully refuted, though they are more than sufficiently convicted by their own conscience; for there is no confidence in God where there is no love of righteousness.

It is indeed true that we are justified in Christ through the mercy of God alone; but it is equally true and certain that all who are justified are called by the Lord, that they may live worthy of their calling. Let the faithful then learn to embrace him, not only for justification but also for sanctification, as he has been given to us for both these purposes, lest they tear him apart by their mutilated faith.

But if you by the Spirit, etc. He moderates his address in this way so that he might not discourage the minds of the godly, who are still conscious of much weakness. For however we may still be exposed to sins, he nevertheless promises life to us, provided we strive to mortify the flesh. He does not strictly require the destruction of the flesh, but only instructs us to make every effort to subdue its lusts.