I have a soft spot for the topic of people who are dual faith. It’s weird, you know. If you’re an atheist, you get a thumbs up from me. If you’re religious with one faith, you get a raised eyebrow from me. And if you are dual faith, you get two thumbs up from me. It just feels like you’re more open-minded if you are more than one faith.
https://en.m.wikipedia.org/wiki/Religious_syncretism
Basically, people combine religions way more than they pretend to. Think about a holiday like Christmas/Yule. It’s allegedly a Christian holiday, but somehow there are elves, reindeer, evergreen trees, and gift giving involved.
Throughout history, religious conversion wasn’t really about personal belief. There have been political conversions where a leader has “converted” for an alliance or marriage, and therefore a whole kingdom is allegedly a new religion, even though the people are still doing what they always have.
https://en.m.wikipedia.org/wiki/Christianization_of_Kievan_Rus’
There have been crusades, Muslim conquests, and colonization forcing conversion at the end of a sword. People kept practicing their existing religions during all of these.
Eventually, public and private religions, religions of neighboring people, etc. blur together.
https://en.m.wikipedia.org/wiki/Folk_Catholicism