John Calvin Commentary Romans 5:21

"that, as sin reigned in death, even so might grace reign through righteousness unto eternal life through Jesus Christ our Lord." — Romans 5:21 (ASV)

That as sin has reigned. As sin is said to be the sting of death, and as death has no power over people except on account of sin, so sin executes its power by death; it is for this reason said to exercise its dominion in this way. In the last clause, the order of the words is rearranged, yet not without reason.

The simple contrast might have been formed in this way— “That righteousness may reign through Christ.” But Paul was not content merely to oppose one contrary with another; instead, he adds the word grace, so that he might more deeply impress this truth on the memory—that the whole is to be ascribed, not to our merit, but to the kindness of God. He had previously said that death reigned; he now ascribes reigning to sin, but its end or effect is death. And he says that it has reigned, in the past tense; this does not mean it has ceased to reign in those who are born only of flesh, and in this way he distinguishes between Adam and Christ, assigning to each his own time. Therefore, as soon as the grace of Christ begins to prevail in anyone, the reign of sin and death ceases.