Florentine Dominican, Riccoldo da Monte Croce, was a thirteenth-century missionary and apologist to Islam who travelled throughout the Middle East during a time of ferment in the Latin Church. While his initial impressions and observations of Muslim spirituality and devotion were largely positive, they altered radically when he observed, in Baghdad, a parade of booty resulting from the fall of Acre to the Mamluk Turks in 1291. Thrown into a theological and spiritual crisis, he searched for a theodicy by which this seeming triumph of Islam over Christianity might be made comprehensible. Upon his return to Florence, he wrote his great work, the Contra legem Sarracenorum (CIS), which was not intended to be constructive but, as he put it, “to expose what is deficient in the teachings of the Saracens” for the benefit of future missionaries from his and other Christian orders. Thus, while Riccoldo’s pre-1291 writings are marked by an open and respectful attitude toward Islam, the CIS represents a systematic and vitriolic attack against it. In the CIS, he takes up the questions of God’s essence and Muhammad’s prophethood and compares Christ with Muhammad and the gospels with the Qurʾan. In the centuries to follow, both Nicholas of Cusa and Martin Luther relied heavily on Riccoldo’s influential work.