For in that he died, he died unto sin once: but in that he lives, he lives unto God.
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Ambrose of Milan
AD 397
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.”
Paul shows that in the Savior’s resurrection we have the assurance of eternity, to which we shall attain if we live a better life. For whoever lives to God by doing good is truly alive here and now and has eternal life. Commentary on Paul’s Epistles.
Paul is saying that if Christ had died for sinners two or three times, there would be no danger in going back to our old sinful ways. But as he only died once, we who have been buried and risen again with him will not die to sin again. There will be no second baptism, no second death of Christ. Therefore we must be careful to stay alive. .
What does “died to sin” mean? It means that he was not subject to sin but that, in order to destroy it and remove its power, he died for our sin. Do you see how Paul frightens them? For since Christ does not die twice, there is no second washing, so you had better steer clear of any inclination toward sin! Homilies on Romans