Anselm's theory on atonement is that of a philosophical one which he believes should be understood as a necessity. Anselm's doctrine, Cur Deus Homo' which is translated as why God became a man (human being)', and the motive behind this work is to prove God's existence. His main target audience are those among the non-believers of God.
Anselm's argument and idea on the incarnation was to build upon the Chalcedonian definition of faith. So Jesus is to be recognized in two natures as he is in the Chacledonian creed. So Anselm sets out to prove the philosophical necessity of the incarnation and the truth regarding the Chalcedonian creed and the nature of Jesus Christ. This next passage, which is one of the most famous from the preface of Cur Deus Homo, explains what Anselm believes to be the absolute reason for the Incarnation of Jesus Christ
"and, in fine, leaving Christ out of view (as if nothing had ever been know of him), [Cur Deus Homo] proves, by absolute reason, the impossibility that any man should be saved without him." So to be precise about what Anselm is implying is that for reconciliation of God and mankind, the incarnation of Jesus Christ is essential. This has been called the ransom theory' or the substitutionary atonement. This is Anselm's objective theory.
Anselm believes that because mankind are the people indebted to the devil but is also indebted by an infinite debt, only God can make satisfaction'. However it should be mankind who pay this debt so Anselm leads us to understand, by reason, that God has to incarnate himself in the form of a man ie Jesus Christ to pay off this infinite debt. So the reconciliation of God and mankind is to be understood as...