Cur Deus Homo is, simply put, St. Anselm's attempt to justify on behalf of God why Jesus had to atone for mankind's sins.

St. Anselm was an eleventh-century archbishop of Canterbury whose Cur Deus Homo addresses itself to the nature of sin and atonement. His treatise uses the form of a dialogue (in the Platonic tradition) between himself and one of his students, Boso. Anselm's work (written towards the end of his life, between the years 1094-8) became significant for its achievement of an atonement theory that was acceptable to contemporary as well as subsequent church fathers.

When Anselm discusses the act of sin, he describes man as incurring a debt against God. According to Anselm, it was impossible for God to pass over indiscretions in his kingdom; if sin was not not paid for or punished, it was not subject to law. The idea that there is no law operative in God's kingdom is heretical in church doctrine—thus, Anselm justifies God's need to exact restitution from man.

Anselm delineates the nature of sin as an act of robbing God of justice. Because this robbery had infinite value, there needed to be someone equal to God, (i.e., Jesus) to execute this atonement.

Anselm also acknowledges that it would have been impossible for man to pay off this debt of sin (being mortal and not divine) without sacrificing himself by spending eternity in Hell. Therefore, only a being who both shared in God's divinity (Jesus) and was of mankind had to atone for mankind's sin.

In addition to discussing the "quid pro quo" nature of this repayment, Anselm also avers that Jesus died willingly (a point on which the scripture is ambiguous at best). Anselm reasons that Jesus had free will not to comply, but nevertheless allowed himself to die for mankind's sins, thus proving his willingness to do so.

Anselm's view became known as the "satisfaction theory" or (formerly) the "commercial theory." The marketplace language of "commercial theory" was ill-fitting to the discussion of theology in the view of the post-eleventh-century Church reformers. After the Reformation, the title of "commercial theory" became known as the "satisfaction theory." The standards of justice and mercy replaced the (comparatively base) idea of money and debt.