Cur Deus Homo

by St. Anselm of Canterbury

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I have been often and most earnestly requested by many, both personally and by letter, that I would hand down in writing the proofs of a certain doctrine of our faith, which I am accustomed to give to inquirers; for they say that these proofs gratify them, and are considered sufficient. This they ask, not for the sake of attaining to faith by means of reason, but that they may be gladdened by understanding and meditating on those things which they believe; and that, as far as possible, they may be always ready to convince any one who demands of them a reason of that hope which is in us. And this question, both infidels are accustomed to bring up against us, ridiculing Christian simplicity as absurd; and many believers ponder it in their hearts; for what cause or necessity, in sooth, God became man, and by his own death, as we believe and affirm, restored life to the world; when he might have done this, by means of some other being, angelic or human, or merely by his will. Not only the learned, but also many unlearned persons interest themselves in this inquiry and seek for its solution. Therefore, since many desire to consider this subject, and, though it seem very difficult in the investigation, it is yet plain to all in the solution, and attractive for the value and beauty of the reasoning; although what ought to be sufficient has been said by the holy fathers and their successors, yet I will take pains to disclose to inquirers what God has seen fit to lay open to me. And since investigations, which are carried on by question and answer, are thus made more plain to many, and especially to less quick minds, and on that account are more gratifying, I will take to argue with me one of those persons who agitate this subject; one, who among the rest impels me more earnestly to it, so that in this way Boso may question and Anselm reply.

Book Summary

Anselm's "Cur Deus Homo" (Why God Became Man) is a theological work that uses philosophical reasoning to explain the necessity of the Incarnation and Crucifixion of Christ for human salvation. Structured as a dialogue between Anselm and a questioning monk named Boso, the book develops the "satisfaction theory" of atonement. Anselm argues that human sin constitutes an infinite offense against God's honor, creating a debt that humanity is obligated to pay but incapable of satisfying. He concludes that only a being who is both fully God (to have the ability to pay the infinite debt) and fully man (to have the obligation to pay it on behalf of humanity) could bridge this gap. This God-man, Jesus Christ, by willingly offering his infinitely valuable life, makes perfect satisfaction to God, thus restoring God's honor and making it possible for God to be both just and merciful in forgiving sinners.

The Problem of Sin and the Necessity of Satisfaction

The first part of the book establishes the core problem that requires divine intervention. Boso, representing both believers and unbelievers, questions why an omnipotent God could not simply forgive humanity by an act of will, rather than undergoing the humiliation and suffering of becoming human. Anselm counters that sin is not merely a transgression but an act that robs God of the honor He is due from His creation. For God to simply forgive this debt without payment would be to leave sin unpunished, creating disorder in His kingdom and treating the guilty the same as the righteous, which would be inconsistent with His perfect justice. Anselm argues that every sin, no matter how small, is an infinite offense because it is committed against an infinite God. Therefore, the satisfaction required to restore God's honor must be greater than the entire created universe. He concludes that humanity is trapped: they owe a debt they cannot possibly pay, and God's justice prevents Him from simply ignoring it.

The God-Man as the Only Solution

Having demonstrated that satisfaction for sin is both necessary and humanly impossible, Anselm presents the logical necessity for the God-man. He reasons that since only man ought to pay the debt but only God can pay it, the one who pays must be both God and man. This leads to the requirement of a unique being: perfect God and perfect man united in one person. This God-man must be taken from the race of Adam to properly represent humanity, yet must be without sin himself, as a sinner cannot atone for others. Anselm explains that this is achieved through the virgin birth, where Christ takes on human nature from a sinful lineage but is conceived without original sin. As a sinless being, this God-man does not owe the debt of death. His mortality is not a punishment but a voluntary condition he assumes, giving him the unique ability to offer his life freely.

The Atonement and Its Application

In the final section, Anselm explains how the death of the God-man accomplishes salvation. Because Christ is sinless, his death is not a debt he owes. By willingly choosing to lay down his life—an act of perfect obedience and a gift of infinite value—he offers a satisfaction to God that far outweighs all the sins of humanity. Since Christ, as God, is perfect and in need of no reward, the immense merit generated by his sacrifice can be applied to others. He bestows this merit upon those for whom he died, covering their debt of sin. This allows God to forgive humanity without compromising His justice; the debt is not ignored but paid in full. This framework explains how God can be both perfectly just, in that sin is punished, and perfectly merciful, in that sinners who place their faith in Christ's work are forgiven and reconciled to Him.