Why was Spain so strongly Catholic

Catholic Christianity is by far the largest religion in Spain. According to a study by the Spanish Centre for Sociological Research in 2016 about 70-75% of Spaniards self-identify as Catholic Christians, 3-5% as followers of other faiths (including Islam, Protestant Christianity and Buddhism etc.), and about 20-22% identify as atheists or non-believers. [1] Most Spaniards do not participate regularly in religious worship.