That was kinda the whole idea
Then it has no sense asking how different would be Columbus' voyages, the Armada, Napoleon or the Civil War. The only thing sure is that nothing remotely similar to those things would happen.
The fact is that, though the process of complete Christinization on the Iberian Peninsula could be slow in some regions, it had an early and fast start, paradoxically (is that the word?). The Christianization of the Iberian Peninsula started right after the Christianization of Carthage, made by missionaries like St. Cucufatus. By Constantine' times the number of Christians in Spain was clearly bigger than in Gaul or Britain, perhaps even bigger than in Italy itself. The Dark Ages could reduce or slow the process of Christianization but there is no way the Iberian Peninsula could return to Paganism the way Britain did. It's simply too rich, too populated, to well placed right near the core of European Christianity that it won't happen. Spain could change Catholicism by other version of Christianity like Arianism or (early) Islam, but not return to long time defunct native gods and beliefs (except for some reduced spots in the mountains of the north that never abandoned them, of course).
So, to achieve that, you either need
a) No Christianily
b) Christianity not gaining popularity in Rome
c) Reduced Roman Empire
d) No Roman Empire at all
Any of these cases would result in thousands of possible scenarios, none of them anything close to our current one. There is no way Spain would be a Pagan nation in a Christian/Muslim Mediterranean. It would be in a Pagan world with a big POD in the Ancient Age (assuming Spain becomes a nation at all in those scenarios). So, as I said, it is simply pointless to consider events related to Spanish (or European) History in the last 500 years of OTL.