John Chrysostom

What does "unto sin" mean? It means that He was not subject even to that one, but for our sin, that He might destroy it, and cut away its sinews and all its power, therefore He died. Do you see how he affrighteth them? For if He does not die again, then there is no second laver, then do thou keep from all inclinableness to sin. For all this he says to make a stand against the "let us do evil that good may come. …

Homily on Romans 11 ↗

Theophylact of Ohrid

"For in that He died, He died unto sin once" means: Christ died for our sin, whereas He Himself was not subject to death. "And in that He lives, He lives unto God," that is, He lives by divine power; for He always lives in the power of God the Father.

Commentary on Romans ↗

Ignatius of Antioch

Mindful of him, do ye by all means know that Jesus the Lord was truly born of Mary, being made of a woman; and was as truly crucified. For, says he, "God forbid that I should glory, save in the cross of the Lord Jesus." And He really suffered, and died, and rose again. …

Epistle of Pseudo-Ignatius to the Tarsians ↗

Ambrose of Milan

In the Scriptures we learn that there are three kinds of death. The first is when we die to sin and live to God. Blessed is that death which, escaping from sin and devoted to God, separates us from what is mortal and consecrates us to him who is immortal. The second death is the departure from this life.… The third death is that of which it is said: "Let the dead bury their dead."

On the Death of Satyrus 2.36 ↗

Thomas Aquinas

Then when he says, for in that he died to sin, he proves his statement, namely, that the risen Christ will not die again. And he proves this in two ways. First he proves it with a reason based on the death he underwent, saying: for in that he died to sin, he died once. This must not be taken to mean that he died to sin he committed or contracted, because sin had no place whatever in him: who did no sin (1 Pet 2:22). …

Commentary on Romans ↗