The central theme in Cur Deus Homo is atonement (the reconciliation of humanity to God), which in turn can be divided into a number of subsidiary themes. Anselm brings a high degree of originality to his treatment of two of these: The first involves the question of humanity’s subjection to the devil; the second, the “necessity” whereby God became human. In the early part of the work, Anselm disposes of the traditional view that God became human because the sin of Adam had delivered the human race to Satan. In this understanding of atonement, the devil held “rights” over humanity that God was obligated to recognize. The Incarnation was seen as a kind of divine trickery because in conspiring to bring about Christ’s death, the devil forfeited his right of possession over humanity. Anselm’s refutation of this notion of Satan’s “rights” established what would be the cornerstone of his argument throughout Cur Deus Homo: The Fall of the human race in no way diminished God’s dominion over his creation. To accept the view that the devil possessed any “rights” whatsoever would have been to accept a compromised idea of God’s omnipotence. Both faith and a rigorous reason demanded that the devil be reduced to a minor player in the drama of atonement.
In diminishing the devil’s role, Anselm places atonement on a new doctrinal basis, one in which the central theme became the story of how God’s mercy is reconciled to his justice. In Anselm’s aesthetic vision, the perfectly ordered universe must reflect a flawless harmony of divine attributes: a perfection of power, justice, order, and beauty. If any of these attributes is diminished in the slightest degree, the harmony of the whole is distorted. Therefore, while God is a merciful God, an arbitrary act of divine mercy that failed to accord with God’s justice would be impossible. Even in his will to forgive, God cannot, of “necessity,” act inconsistently against his own rectitudo. To find scope for his mercy, then, it was “necessary” that God become human and be punished for humanity’s sin. For justice and for the universal harmony to be restored, compensation had to be made. Therefore Anselm introduced into the theological tradition a rational coherence that would shortly give rise to what is known as Scholasticism.