Religion and war are in the game and it does make your realm better to have an army of priests doing your bidding the same as soldiers. Nothing is stopping the Astroists from conducting mass auto-de fes in your lands you have nothing to fight back against the religions of Dwilight decimating your lands with our preaching.
Aside from the fact we'd arrest them the moment they arrived, we're pretty good at border control, as we've shown on multiple occasions when priests have tried to do exactly that. Oh, and the indigenous religions, and the foray of foreign religions that crept into our realm that ensures no one has a direct majority and to gain one would require sufficient time for us to deal with a priest.
It's not like we haven't considered such a situation and considered what we would do to when it occurs, and it's not like we haven't handled such situations in the past.
