Except that in BM, you have a ton of religions bordering each other everywhere. So since "border regions" mixed faiths a lot (relatively) in the real world, and that almost all realms in BM share a border with a realm of another faith, it's hardly surprising that people are so often soft on the issue. Other than Sanguis Astroism, I don't think any one religion has ever gained a significant contiguous spread so that any realm would be surrounded by nothing but fellow faithful.
Magna Aenilia Ecclessia managed it at one point, though it's a fairly poor example given the "theology" of the religion. The realm of Greater Aenilia was surrounded by Arcaea, Ethiala, Cathay, and Zonasa, and MAE had a strong presence if not dominance in all of those realms.
