Why God Became Man (Cur Deus Homo)

A dialogue in two books between Anselm and his pupil Boso, finished in 1098 while Anselm, now archbishop of Canterbury, was in exile from England. It asks why God became human to save humankind, and tries to answer by reason alone, setting Christ aside as if nothing were known of him. Book I rejects the older view that the devil had rights over humanity. Sin is failing to give God the honour owed to him, and justice requires either punishment or satisfaction, a repayment greater than the wrong. But humans already owe God everything, so they have nothing extra with which to pay, and God's purpose in creating them would fail if none were saved. Book II concludes that only God can make the satisfaction, while only a human ought to make it, so it must be made by one who is both, freely giving up a life he did not owe. Critics argue that the scheme depends on feudal ideas of honour and debt.

It is the classic statement of the satisfaction theory of the atonement, and a model of tightly argued medieval dialogue for anyone thinking about justice, debt and forgiveness.