Well, you just need to stop Aragon from inviting the Byzantines in Sicily in to help with the Reconquest of Hispania from the Muslims. Once the Byzantines had a presence in Aragon, they slowly built their power and were able annex the state within a few generations and the rest, as they say, is history. Byzantine armies marching against the Islamic states of the Muslim principalities and then turning against their Christian neighbors as well. It seems logical to us now, only because it happened in our history - I'm pretty sure any other timeline would view the events (a Greek Empire in the east conquering a huge peninsula in the west from a staging ground in Sicily) was borderline ASB to say the least!
Amyway, without those events, the Christian states in Spain stay firmly in the camp of the Pope. Assuming they are still able to drive the Islamic states out of the peninsula - and I don't see why they wouldn't be able to - you have Catholic Spain.