The two most famous theologians from the Medieval period are Anselm and Thomas Aquinas.
Bishop of Canterbury, Anselm is famous for two important theological contributions.
1) First, in Proslogion, he presented what has been called the "ontological argument" for the existence of God. This argument is as follows: a) God is "that than which nothing greater can be conceived." b) If He only existed in thought and not in reality, then He would no longer be "that than which nothing greater can be conceived," because a similar being that exists in reality would be greater. c) Therefore, God ("that than which nothing greater can be conceived") must exist in reality.
2) Secondly, in Cur Deus Homo (Why the God Man?), he gave a concise explanation of the substitutionary atonement. He explained that God had to become man in Jesus Christ in order to pay the punishment for sin as our substitute. God had to punish man for his sin, because He is just. But God also desired to save His people, because He is merciful. Therefore, instead of punishing us, He punished His own Son in our place. Jesus had to be man in order to take our place. But He also had to be God, and without sin, so that His sacrifice would be accepted as perfect and satisfactory. This interpretation has been accepted as the traditional orthodox view of the atonement.
