Anselm's Cur Deus Homo, Why God Became Man, finished in exile around 1098, asked the question its title states and answered it with a new account of the atonement. The older ransom theory had pictured the crucifixion as a price paid to the devil, who held humanity captive by right of conquest. Anselm dismissed the devil's rights entirely. Sin, he argued in the feudal vocabulary of honor and obligation, is the creature's failure to render God what is owed; it leaves a debt that justice cannot simply waive. Humanity owes the debt but cannot pay it; God can pay it but does not owe it; therefore only one who is both God and man can make the satisfaction, which is why God became man, and why the cross is neither a trick played on the devil nor an arbitrary display, but the necessary repair of a moral order.
The argument, restated and modified, became the trunk of Western atonement theology. Aquinas softened its necessity, the Reformers sharpened satisfaction into penal substitution, Abelard's moral influence account defined itself against it, and modern theology still stakes positions by their distance from Anselm. The atlas records the satisfaction theory as a doctrine in its own right, attributed to this book; few works of eight hundred pages have organized a field the way this short dialogue organized centuries of thought on why, in Christian terms, the death of Christ saves.