When did the Spanish decide to have Catholicism as their religion?

Answer 1

The Visigoths, who practised a form of Christianity known as Arianism, received Roman Hispania (the Iberian Peninsula) and Southern Gaul in the 5th century CE as a payment for their loyalty to the Roman Empire. This created a society in which the original inhabitants were Catholics, while the ruling classes were Arian Christians.

At the third synod of Toledo in 589, the ruling Visigoths, who had been Arian Christians, appear to have submitted to the Catholic Church and were obliged to accept the Nicene Creed with the filioque. However, after several centuries, Count Julian of Ceuta called for the Muslims (or Moors) to enter Hispania, because the king had converted to Catholicism and insisted that his subjects do likewise. The rebels considered Islam a related religion as opposed to heresy of Trinitarian Catholicism. By and large, the Moors treated the Spanish Christians and Jews well, and allowed them to follow whatever creed they wished.

Between 718 and 1492, Christian armies slowly began pushing back Muslim control. The Christian invaders were Catholic, so Spain 'decided' to be Catholic as the reconquest proceeded. The Spanish Inquisition was set up to investigate conversos, secret Jews who had claimed to have converted to Christianity, but after 1530, turned its attention to ensuring that Protestantism would not infiltrate Spain as it had Germany and France.