Order of Elders is one of the largest religions on FEI. It doesn't control any realms outright, but it is very widespread, and has quite a few noble members who are powerful in their own right.
While there are a few large religions on FEI, they mostly shy away from taking any direct involvement in politics. Realms usually go in big for the "separation of church and state" schtick. Except for the Sartanians, of course. And they often seem like a Three Stooges kind of act.