John Calvin Commentary Romans 6:8

"But if we died with Christ, we believe that we shall also live with him;" — Romans 6:8 (ASV)

But if we have died, and so on. He repeats this for no other purpose than to add the explanation that follows: that Christ, having once risen, dies no more.

By this, he teaches us that newness of life is to be pursued by Christians as long as they live. For since they should represent in themselves an image of Christ—both by crucifying the flesh and by a spiritual life—it is necessary that the former be done once for all, and that the latter be carried on continually.

This does not mean, as we have already said, that the flesh dies in us in a moment, but rather that we should not regress in the work of crucifying it. For if we roll again in our own filth, we deny Christ, of whom we cannot be partakers except through newness of life, since he lives an incorruptible life.