Anselm of Canterbury, answered from the texts and cited to the page.
Sin is the failure of a rational creature to render to God the obedience and honor that is owed. By sinning, human nature dishonored God; the dishonor must be repaid before forgiveness can be granted consistently with God's justice. The Augustinian devil-ransom theory — that Christ's death paid a debt to Satan, from whose dominion humans were thus ransomed — is mistaken: Satan has no rightful claim against God's creature; God owes Satan nothing.
What is owed is owed to God, by humans. Yet humans, in their fallen state, cannot pay the debt — for any obedience humans might render is already owed to God, and so cannot constitute additional satisfaction for past offense. Only one who is fully human (and so owes the debt) and fully God (and so can offer something not already owed, namely a sinless life freely surrendered) can pay the debt of honor.
Therefore the incarnation was necessary: Cur Deus Homo — why God became man. The God-man's voluntary death, undeserved because the man was sinless, constitutes a payment to God's honor of such surpassing worth that it outweighs the offense and makes salvation available to all who repent.1234
Boso. I grant it; for in these there is nothing which seems unbecoming or impossible for God. Anselm. Therefore, in order that man may attain happiness, remission of sin is necessary. Boso. We all hold this.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo, pp. 201–203
Boso. I am wholly unable to refute your reason ing. But what say you to this: that we pray God, "put away our sins from us," and every nation prays the God of its faith to put away its sins. For, if we pay our debt, why do we pray God to put it away? Is not God unjust to demand what has already been paid?Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo, pp. 225–226
In fine, you have shown that that man, who was also God, must be formed from the virgin, and from the person of the Son of God, and that he could be taken without sin, though from a sinful substance. Moreover, you have clearly shown the life of this man to have been so ex cellent and so glorious as to make ample satisfaction for the sins of the whole world, and even infinitely more.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo, pp. 279–280
There was nothing, therefore, on the part of the devil to make it due, in respect of him, that God should, having the salvation of man in view, either conceal or set aside His omnipotence. Was there, then, any inherent necessity com pelling the Most High thus to humble Himself, and the Almighty to toil as He did for the attain ment of any end of His?Book of Meditations and Prayers, pp. 140–142
Defended in Cur Deus Homo, written across the years of my flight from England under William Rufus (begun c. 1094, completed in the Alps c. 1098). The book is structured as a dialogue with Boso, my monk-companion, who presses on behalf of those who do not believe — asking why God could not simply forgive, why a non-Adamic sinless man could not have made satisfaction, how Christ's voluntary death can outweigh the sins of all humanity. Each question receives careful argument. The theory has been criticized — Abelard proposed a moral-exemplar alternative in the next generation; Aulen in the twentieth century pressed the ransom-from-the-powers reading against me; modern critics have charged the account with "feudal" legalism. The feudal vocabulary is present, as the vocabulary of any age is present in its theology, but the argument does not depend on the feudal social structure for its force. I stand by the central structure of the argument.