One of the reason why Spain was so strongly catholic was that the very existence of the country ( as in Spain in the possession of the people whose successors are ruling today) can be owed to a religious zeal: the territory was previously under Muslim control and it was taken (back) in the Reconquista. Because people have seen the Reconquista as a religious war to spread the religion, a lot of soldiers from all of Europe came to help, without whom the Reconquista would not be possible. Later when they took over the country and ruled it, they implemented their religious views in the new country, and a legacy of this is the strict religiousness of the Spaniards in the past
