Pope Sixtus IV granted Isabella and Ferdinand authority over the Spanish Inquisition in Castile via papal bull.
On 13 March 1479, Pope Sixtus IV issued a papal bull granting Isabella and Ferdinand authority over the Spanish Inquisition in Castile. This authorization placed the mechanisms of religious investigation and punishment under unprecedented royal control rather than direct papal supervision.
The bull empowered the monarchs to appoint inquisitors and oversee procedures aimed at scrutinizing the orthodoxy of conversos—Jews and Muslims who had converted to Christianity—and other suspected heretics. Isabella, deeply committed to Catholic unity, adopted the Inquisition as a tool to enforce religious conformity and, indirectly, consolidate political power.
Historically, this event marks a turning point in the relationship between crown and church. The Inquisition, while ecclesiastical in nature, became a semi-state institution aligned with royal objectives, contributing to persecution, forced conversions, and executions that scarred Spanish society.
From a women’s-history standpoint, Isabella’s role in securing and directing inquisitorial powers underscores the capacity of a female ruler to intervene decisively in religious policy—illustrating both the breadth of her authority and the morally complex legacy of her reign for minority communities within and beyond Spain.