information:highlighting, information:filtering
The "Keeping the Pope in Check" hypothesis

The hierarchy of the Catholic Church had made many attempts during the Middle Ages to take over Christian Spain politically, such as claiming the Church's ownership over all land reconquered from non-Christians (a claim that was rejected by Castile but accepted by Aragon and Portugal). In the past, the papacy had tried and partially succeeded, in forcing the Mozarabic Rite out of Iberia. Its intervention had been pivotal for Aragon's loss of Rosellon. The meddling regarding Aragon's control over South Italy was even stronger historically. In their lifetime, the Catholic Monarchs had problems with Pope Paul II, a very strong proponent of absolute authority for the church over the kings. Carrillo actively opposed them both and often used Spain's "mixed blood" as an excuse to intervene. The papacy and the monarchs of Europe had been involved in a rivalry for power all through the high Middle Ages that Rome had already won in other powerful kingdoms like France.

Since the legitimacy granted by the church was necessary for both monarchs, especially Isabella, to stay in power, the creation of the Spanish Inquisition may have been a way to apparently concede to the Pope's demands and criticism regarding Spain's mixed religious heritage, while at the same time ensuring that the Pope could hardly force the second inquisition of his own, and at the same time create a tool to control the power of the Roman Church in Spain. The Spanish Inquisition was unique at the time because it was not led by the Pope. Once the bull of creation was granted, the head of the Inquisition was the Monarch of Spain. It was in charge of enforcing the laws of the king regarding religion and other private-life matters, not of following orders from Rome, from which it was independent. This independence allowed the Inquisition to investigate, prosecute and convict clergy for both corruptions and possible charges of treason of conspiracy against the crown (on the Pope's behalf presumably) without the Pope's intervention. The inquisition was, despite its title of "Holy", not necessarily formed by the clergy and secular lawyers were equally welcome to it. If it was an attempt at keeping Rome out of Spain, it was an extremely successful and refined one. It was a bureaucratic body that had the nominal authority of the church and permission to prosecute members of the church, which the kings could not do, while answering only to the Spanish Crown. This did not prevent the Pope from having some influence on the decisions of Spanish monarchs, but it did force the influence to be through the kings, making direct influence very difficult.