The Tribunal of the Holy Office of the Inquisition was authorized by Pope Sixtus IV in 1478 and the first inquisitors, Miguel de Morillo and Juan de San Martín, were appointed by the future Catholic Monarchs, King Ferdinand II of Aragon and Queen Isabella I of Castile, in 1480. Although its stated aim was to maintain Christian orthodoxy, it became an effective instrument of state power by replacing the Medieval Inquisition, which was under Papal control.