Had it not been for Brude Donnahue, the kingdoms of the Human World would still require the Divine Court to appoint their kings.
The true purpose of this man, who instigated the war among the gods, was to encourage humanity to become stronger.
Humans could admire and worship the deities, but they must not lose their sense of self.
He succeeded... but not completely.
More than two thousand years later, most kingdoms in the Human World still pay taxes to the temple every year.
As for those principalities, it's even worse; the temple can directly appoint the heir to a principality.
Still, you can't really call the temple the antagonist.
The temples capable of appointing heirs to principalities carry the duty to protect them. When a principality faces war, or if a town is invaded by foreign races, savage beasts, or magical beasts, the temple will send knights to assist the principality's soldiers in defending the city.
There are good and evil clergy within the temples.