Doubt is a hesitation, an uncertainty. Here we propose considering this concept from the perspective of Catholic doctrine. Our main concern will be the reflexion of Jesuit Juan Azor (1536–1603) on doubt in his Moral Institutions (Rome, 1600). Azor gives his own definition of the crime of heresy and asks the question: “Is he who hesitates in the faith a heretic?” (An dubius in fide sit hereticus.) Before giving his own opinion, he refers to six writers who have dealt with the subject before him. These texts, which form the body of this study, trace a collective route under the terms of which the Jesuit records his own solution. (...) Over the generations, the collective work of doctors has highlighted a set of attitudes vis-à-vis the claims of faith, from firm adherence to total refusal, to the desire to understand and to demonstrate, the fleeting temptation of disbelief, amazement at miracle and mystery, or distrustful distancing from the Church’s statements. Hesitation, which at first seems innocent because of its very vagueness, does not stand up to scrutiny. Doubt, when settled permanently in the individual consciousness without being fought by the forces of the will, is a sign of disobedience to the Church and an inner resistance to the gift of faith. It could therefore only be condemned by defenders of Catholic orthodoxy, who saw this as the door opening to heresy and atheism.