Cur Deus Homo (""Why God was a Man"") was written from 1095 to 1098 once Anselm was already archbishop of Canterbury and discusses the Incarnation. It takes the form of a dialogue between Anselm and Boso, one of his students. Its core is a purely rational argument for the necessity of the Christian mystery of atonement, the belief that Jesus's crucifixion was necessary to atone for mankind's sin.