Okay having messed around with the religion some more I'm definitely finding it a compelling game mechanic. But I was confused about it and after reading discussions about it on discord it seems pretty bad. If there are other religions in a region following your state region it gains +20 stability. Usually a dominant religion eventually excludes other religions. Maybe ... maybe if you're really struggling with instability it might get you through a rough patch of the game. But adopting a religion can cause instability so it's basically moot. Should the tier 1 and tier 4 stability tenets  be swapped? Not every city will be on a river but most will have sections or territories with rivers for food production. 

This order seems a lot better, both in valuation and impact at the time you pick it.

Tier 1 - Sustain the faithful.

Tier 2 - Protect the Weak
Tier 3 - Be Virtuous as Water
Tier 4 - Proselytize Daily