Fortalitium fidei is one of the central texts in the controversy surrounding the religious and social status of conversos in fifteenth-century Castile. This monograph provides a close analysis of the text itself and contextualizes this study through comparison with pro-converso texts and with reference to Alonso de Espina's career as an Observant Franciscan. After an outline of the development of the converso problem, it offers a biography of Espina and a discussion of the context of production of Fortalitium fidei. There is then a discussion of three works of theology in defence of conversos: Alonso de Cartagena's Defensorium unitatis christianae, Juan de Torquemada's Tractatus contra madianitas et ismaelitas, and Alonso de Oropesa's Lumen ad revelationem gentium. The rest of the work is detailed reading of Fortalitium fidei, with chapters on the image of the fortress, the treatment of Jews and Judaism, and of conversos. This volume addresses the extent and nature of the debate about conversos, the development of models of genealogical exclusion, and the role of Espina and his text in the ending of religious plurality in Spain.