원죄
This is a concept similar to Anselm's that we owe a debt of honor to God, with a critical difference: While Anselm said we could never pay this because any good we could do was owed to God anyway, Aquinas says that in addition to our due of obedience we can make up for our debt through acts of penance "man owes God all that he is able to give him...over and above which he can offer something by way of satisfaction". Unlike Anselm, Aquinas claims that we can make satisfaction for our own sin, and that our problem is not our personal sin, but original sin. "Original sin...is an infection of human nature itself, so that, unlike actual sin, it could not be expiated by the satisfaction of a mere man." Thus Christ, as the "second Adam," does penance in our place – paying the debt of our original sin. Why does he do that? By love. The whole of the work of redemption begins with God's love: “God so loved the world that he gave his only Son” ().