Why were Catholic monarchs called by this name?

Catholic monarchs were called by this name because they were staunch defenders of the Catholic faith and played a significant role in promoting and protecting Catholicism in their realms. The term is most commonly associated with King Ferdinand II of Aragon and Queen Isabella I of Castile, who united Spain in the late 15th century and sought to consolidate religious unity by supporting the Spanish Inquisition and expelling non-Christians from their territories. Their devotion to the Church and efforts to spread Catholicism earned them the title from Pope Alexander VI in 1496.