First time building a world

I wouldn't say this is contrived at all. You have a world with five societies that are based around a central religion, which is very plausible, especially with a good geography. It sounds like each of the 5 factions would be relatively similar in size to one another before the tides are turned. It also makes a lot of sense that each faction would have a leader, or counsel, that was thoroughly convinced that their gods were the best ones to follow and that the other four religions in the world were a threat to their gods' existence. Heck, you could even have it where each of the religions' gods were real and all at war among one another and using the five societies as pawns in that war since they couldn't directly interfere. Perhaps the gods' power grew greater the more followers they had, etc, which would give them incentive to do what you planned. One of the gods, in this case, could have found a way to sway a Mastermind beholder into doing what you suggest, which sound very compelling.

In the end, it sounds like the normal people of the world, those most affected by war, were mostly on board with whoever the victor was, simply because war sucks for the life of a peasant, so they ended up being happy enough to quell any sort of alternate religious fervor simply in fear that war would spark up again. If the god who one is an evil god, however, then they may also be living in fear of that god's wrath, and reluctantly worship him while secretly planning a rebellion or something, as they may be living life feeling like prisoners.