One time I was the founder of Christianity and all my cities were Christian but then somehow one of my cities became Buddhist. I don't mean it was Christian then became Christian and Buddhist but it became only Buddhist. I have never been able to do this to other cities or even my own, to not add a religion but completely replace the current one. Possibly even wierder was that I was in a theocracy so non Christianity shouldn't have spread. It might be a bug but I sort of hope not I would like to be able to do it, does anyone know how?