Actually, this would give strong chance to the idea of a passive religious realm. Deploy militia, and declare any trespassers anathema. Might not fight off huge invasions, but could be feasible for more even matched realms.
For example, Shadowism on the Colonies has a lot of priests, and ties to the Assassins and Minas Thalion. Lukon is trying to setup s city-state empire in the Colonies. Should a small state, say Wetham, try to invade Shadowism lands they'd get their asses handed to them..... kind of silly.
