"But the debt was so great that while man alone owed it, only God could pay it, so that the same person must be both man and God. Thus it was necessary for God to take manhood into the unity of his person, so that he who in his own nature ought to pay and could not should be in a person who could." - Anselm of Canterbury in "Cur Deus Homo" ("Why God Became Man")

Christians firmly believe that Jesus was both fully God and fully man. Anselm presented a case for the atonement in his work "Cur Deus Homo," bringing the doctrines of the Incarnation (Jesus, the Son of God, taking on humanity) and Crucifixion (Jesus, the Son of God, dying for humanity) together. Only Jesus could be the perfect sacrifice and substitute. Only he could pay for our sin debt and satisfy the wrath of God. "For there is one God, and there is one mediator between God and men, than man Christ Jesus, who gave himself as a ransom for all, which is the testimony given at the proper time." (2 Timothy 2:5-6)