John Chrysostom

This he says to show that the latter ranks as a king, the former, death, as a soldier, being marshalled under the latter, and armed by it. If then the latter (i.e. sin) armed death, it is plain enough that the righteousness destructive hereof, which by grace was introduced, not only disarms death, but even destroys it, and undoes entirely the dominion thereof, in that it is the greatest of the two, as being brought in not by man and the devil, but by God and grace, and leading our life unto a goodlier estate, and to blessings …

Homily on Romans X ↗

Theophylact of Ohrid

Having said that grace appeared superabundantly, the apostle, lest we be faithless, shows that such a manifestation of it corresponds to its purpose, and says: sin was the king, and death was the soldier armed by it. If sin reigned over us, having death as a kind of soldier, then all the more will grace reign in us, which imparts righteousness that destroys sin, and together with the destruction of sin also destroys death, and after the latter, justification. …

Commentary on Romans ↗

Thomas Aquinas

Then when he says that as sin has reigned to death, he shows the effect of abundant grace, an effect that corresponds by way of opposition to that of sin. That as sin, introduced by the first man and abounding through the law, has reigned, i.e., obtained complete dominion over men, and this until it brought them to death both temporal and eternal: the wages of sin is death (Rom 6:23), so also grace, i.e., God's grace, might reign, i.e., rule entirely in us, by justice, which it produces in us: they are justified by his grace (Rom …

Commentary on Romans ↗