This thesis analyzest he development of the contvrspor oblem during the reign of Enrique IV of Castile (1454-1474) through the study of a work of religious polen-ýc entitled Fortalitiumfidei, Written by the Observant Franciscan Alonso de EspMa between 1458 and 1464. Chapter 1 studies the life and background of the author drawing a picture of his social network within the Observant branch of the Franciscan Order and the royal court. It also identifies the main purpose of the Fortalitiumfidei as a collection of sources for preachers. Chapter 2 provides a study of the structure and arrangement of materials in the Fortalitiumfidei, the text provides a warning to society of the danger posed by its non- Christian enemies (heretics, Jews, Muslims and demons) and proposes a series of remedies to deal with this problem: first the reform of Christian society and second the isolation and elimination firom its midst of its enemies. Chapter 3 studies the depiction of conversoins particular, arguing that it creates a group identity for them as judaizing heretics. They are placed in a separate category from Christians and the establishment of a Papal Inquisition is proposed to investigate their activities. Chapter 4 places these findings in its historical context, arguing that the Fortalitium fidei contributed to the development of the converspor oblem by placing notions about the discrimination of neophytes within an orthodox context.