The only way in Spain is to alter the Reconquista. If you can make the Convivencia stronger than it later became, you can see Protestantism as just another of many variations along with Jewish, Muslim (sunni and slightly different Berber versions), Roman, Mozarabic and in some rare cases Celtic rites. Lutheranism might be your best bet here as it has several similarities compared to the Calvinist/Anabaptist styles.
I'd say the best way to do that is to have a quicker, easier Reconquista (like in my own TL) that forces the Spanish kings to deal with a much larger number of muslims in general in their regions early on, and prevent the setbacks dealt by the Africans. The Spanish first started to become Radical Catholics via a combination of the psychological effects of the Crusades and the success of the North African Empires in Al-Andalus. Reduce the success of the Almoravids (especially) in some way or create a stronger Byzantine Empire for instance that might prevent the southern crusades all together. If you can keep the religious diversity in the peninsula, the king might feel unable to suppress non-catholicism effectively or see it as too harmful to the state and might have to adopt religious freedom as a matter of survival.
I don't think it's possible to have a Lutheran dominated kingdom with the political structure of Spain. There's a reason that England is the only one of the "major" states that flipped.