The question is what exists capable of filling that gap. Just answering "paganism" won't suffice. The population itself looks for a state-approved religion and Catholicism/variants will fill that gap.

There is a priest on each village, there is a top-down organization making sure that all rites are properly executed. Where is the European answer to that? I don't think none of the rulers around here have the will nor motivation to move away from desert tribe religions.