Thirteenth-century mendicant missionaries deployed rationalistic arguments to attempt to prove the irrationality of Islam. Yet at the same time, the works of Muslim philosophers, scientists and theologians became an integral part of the curriculum of European universities. This posed a problem: if Islam is as "irrational" as the polemicists and missionaries claim, how could the authors of such sophisticated works of learning adhere to its doctrines? This article examines the response to this problem offered by various medieval writers, in particular four writers of the thirteenth century: Ramon Martí, Roger Bacon, Ramon Llull, and Riccoldo da Montecroce. These authors claimed that learned Saracens did not in fact believe in the doctrines of the Qur'ān, that only the fear of physical punishment made them publicly proclaim their adherence to Islam. All four Christian polemicists were well read in Arabic philosophy; they base their claims on their (mis-)reading of Ibn Sinā, al-Ghazāli, and Ibn Rushd. The (real) philosophical and theological disagreements between Muslim thinkers become "proof" of the "irrationality" of Islam.