St. Anselm, Archbishop of Canterbury in England, wrote Cur Deus Homo ("Why God [Became] a Man") in the eleventh century to introduce and clarify his theology to the public. The treatise propagated what became doctrinally known as a "satisfaction" view of atonement, which stated that Christ suffered crucifixion to atone for mankind's sins and to satisfy God. The treatise is divided into two books, each comprised of several dozen chapters. Throughout the treatise, Anselm uses an interlocutor, Boso (one of his students), to delineate his theories.

Book 1 begins by explaining the central question for his contemporary readers: What proof is there that God became a man? Anselm acknowledges that his proofs should rely on reason as well as the acceptance of the majority. Anselm says that it is fitting that Christ was crucified on a tree, as the devil conquered man by means of a tree. Anselm contends that only God (and not another minor angel) could be the one to deliver Himself to man, as it puts man in debt to Him.

Anselm then discusses the relative claims on mankind by the just God and the unjust Satan. Anselm also entertains the objection that the Son of God (Jesus) died unwillingly, but claims that God had no other way to restore the human race. Moreover, Anselm contends that the Son did in fact will the salvation of man through his death.

Anselm spends much of the first book explaining why this manner of redemption was necessary and reasonable. He holds that man could not repay God for his sins without an act of redemption. Because mankind's sin of being tempted by the devil was so great, there was no other way for man to repay God without a great act of redemption to satisfy God.

In book 2, Anselm explores the alternatives that God had when mankind sinned. He had to either redeem or extinguish mankind, and he chose the former because of the preciosity of mankind. Anselm claims that this redemption, moreover, had to be accomplished by means of the incarnation of Man, as it would be impossible to accomplish redemption for mankind without an act on the part of man.

Finally, Anselm admits that Jesus—even while he took up the form of man—maintained the identity of the divine. He had the choice to comply or to not comply with his own death; therefore, Anselm argues that he must have died of his own free will.