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How did Pope Sixtus IV authorize the Spanish Inquisition?

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Pedro Berruguete: St. Dominic Presiding at an Auto-da-Fé — inquisition *St. Dominic Presiding at an Auto-da-Fé*, panel by Pedro Berruguete, c. 1503; in the Prado, Madrid.

Pope Sixtus IV authorized the creation of the Spanish Inquisition on November 1, 1478, by issuing the papal bull Exigit sinceras devotionis affectus ("Sincere Devotion Is Required").1

This document granted the Catholic Monarchs, Ferdinand II of Aragon and Isabella I of Castile, the authority to appoint inquisitors to address the perceived threat of heresy within their domains, specifically targeting conversos—individuals of Jewish descent who had nominally converted to Christianity but were suspected of practicing their original faith in secret.21

While the Pope initially provided this ecclesiastical framework, the Spanish monarchy utilized the institution primarily to consolidate royal power and enforce religious and social unity. When the early inquisitors in Seville acted with extreme severity, Sixtus IV attempted to intervene and limit their powers, though these efforts were largely unsuccessful as the Spanish crown viewed the Inquisition as a vital political tool. In 1483, the Pope was further induced to authorize the appointment of a grand inquisitor for Castile, effectively extending the reach of the Inquisition across the Spanish kingdoms.2