“For the devil would not have been able to tempt mankind if it had been God’s will to prohibit him. Would God, I repeat, not be acting in a similar way if he were to bring man back to Paradise stained with the filth of sin without any washing, that is, in the absence of any recompense, at least supposing man were to remain in this state for ever?
…Consider it, then, an absolute certainty, that God cannot remit a sin unpunished, without recompense, that is, without the voluntary paying off of a debt, and that a sinner cannot, without this, attain to a state of blessedness, not even the state which was his before he sinned. For, in this case, the person would not be restored, even to being the kind of person he was before his sin.
…It is to no avail that someone who is not making payment says, ‘Forgive’, and the reason why someone who is making payment makes supplication is that this very fact of his supplication is a contingency of relevance to the repayment of the debt. For God owes nothing to anyone, but all creation is in debt to him, and therefore it is not expedient that a human being should deal with God as an equal deals with an equal.”
-Anselm of Canterbury, Why God Became Man, Book 1, 19.