Anselm is the most important Christian theologian in the West between Augustine and Thomas Aquinas. His two great accomplishments are his Proslogium (in which he undertakes to show that Reason requires that men should believe in God), and his Cur Deus Homo? (in which he undertakes to show that Divine Love responding to human rebelliousness requires that God should become a man).
He was born in Italy about 1033, and in 1060 he entered the monastery of Bec in Normandy to study under Stephen Lanfranc, whom he succeeded in office, first as prior of Bec, and later as Archbishop of Canterbury.
He also devoted the time to writing a book known as Cur Deus Homo? (meaning Why DID God Become Man?). In it he puts forward the "satisfaction theory" of the Atonement. Man's offence of rebellion against God is one that demands a payment or satisfaction. Fallen man is incapable of making adequate satisfaction, and so God took human nature upon Him, in order that a perfect man might make perfect satisfaction and so restore the human race. The success of his work may be gauged by the fact that many Christians today not only accept his way of explaining the Atonement, but are simply unaware that there is any other way.