Cur Deus Homo by St. Anselm was written in the final decade of the eleventh century. It represents Anselm's more mature work, insofar as the author was already archbishop of Canterbury. In this work, Anselm addresses himself to the task of discussing the nature of the atonement of Jesus for mankind's sin, as well as the dual nature of Jesus as both human and divine. In making his argument, Anselm appeals to reason. The treatise takes the form of a Platonic dialogue, in which Anselm's interlocutor, Boso, is his student and Anselm plays the role of the philosopher.

Anselm sets out in his preface the occasion of his work:

From the theme on which it was published I have called it Cur Deus Homo, and have divided it into two short books. The first contains the objections of infidels, who despise the Christian faith because they deem it contrary to reason; and also the reply of believers; and, in fine, leaving Christ out of view (as if nothing had ever been known of him), it proves, by absolute reasons, the impossibility that any man should be saved without him. (Preface)

In brief, Anselm is responding to the infidels (Jews and Muslims). They naturally questioned the ability of Jesus to be both God and Man, and, furthermore, they question whether God would be just killing someone on behalf of mankind, who had sinned.

Therefore man cannot and ought not by any means to receive from God what God designed to give him, unless he return to God everything which he took from him; so that, as by man God suffered loss, by man, also, He might recover His loss. But this cannot be effected except in this way: that, as in the fall of man all human nature was corrupted, and, as it were, tainted with sin, and God will not choose one of such a race to fill up the number in his heavenly kingdom. (Book 1, chapter 23)

Later in his argument Anselm affirms that Jesus had to be sent to atone for man's sin, because an ordinary man, corrupted, as it were, by the Edenic fall, would be incapable of furnishing sufficient recompense. This sets up Anselm's position for the second book, in which he argues that Jesus Christ was sent by God in the form of man so that he could properly atone for mankind's sin. Only a jointly human and divine figure could accomplish this. On this score, Anselm states:

So our Lord Jesus, when he wished, as we have said, to suffer death, ought to have done precisely what he did; because he ought to be what he wished, and was not bound to do anything as a debt. As he is both God and man, in connection with his human nature, which made him a man, he must also have received from the Divine nature that control over himself which freed him from all obligation, except to do as he chose. (Book 2, chapter 18)

Cur Deus Homo promotes a view that would become known widely as "satisfaction theory" of atonement. This takes its name for the notion it proposes of Jesus being uniquely able to "satisfy" God and so account for mankind's sin. This view became widely popular within Catholicism and other Western Christian Protestant denominations. Part of what makes Anselm so popular is his appeal to reason as well as his consistency with Scripture (viz. the Old and New Testaments). Though Anselm focuses on crafting an argument form reason (without appeal to Scripture), he does this precisely to prove what Scripture attests (that Jesus is the son of God in the form of man). Anselm's theology was consistent with contemporary doctrine, and so satisfied the Church. Anselm closes his argument as follows:

If we have said anything that needs correction, I am willing to make the correction if it be a reasonable one. But, if the conclusions which we have arrived at by reason seem confirmed by the testimony of the truth, then ought we to attribute it, not to ourselves, but to God, who is blessed forever. Amen. (Book 2, chapter 22)