[Taqwus]

If you're not preaching along your border, you probably should. That might help fight the intruding dominion and reduce the risk of massive, essentially unreversible population loss. Another use for excess priests would be "banish/banish/banish/run away!" attacks just to attrit him a bit.
If it's a multiplayer game, a temporary alliance with Pythium might make sense. Presumably neither of you wants your populations completely gone.