In that case the Elder should be a member of the realm for an X number of days. Otherwise you'll get someone being clever, moving a elder into a realm, declare them faithful and then move back to another realm again.
Perhaps a smarter way to solve this is to have the ruler agree (or disagree) with the faithful status (not the evil status of course) of the realm?
I see what your going for on the rulers deciding but I think it will be turned down a lot more than is true because rulers might see that as submitting to the religion or if its a ruler who doesn't like the religion despite the rest of the realm likings.