I believe (sorry had to be) that religion as presented so far, is still wip and it will probably be better to grasp when the game is out of the alpha or beta stage. That said I'm ofc no Dev.

As for what it is and what it does: I understand it like a second influence scale that provides certain boni to you, but also to potential followers. You can concentrate on your own religion, but also try to accumulate a foreign one. The boni granted vary strongly with the tenets picked. Religion founding is pretty easy (I think it was smth like you got 3 pop in your city!?) strengthening your religion is done mostly with buildings that create faith I think. I still need to wrap my own head around the system though. There is certainly someone who can explain it better than me. :)

I have not played the Stadia Opendev (don't have Stadia), but in the current VIP build, religion is considered a work in progress (still some kinks to work out).

HBumblebee is more/less correct. Unless something is different in the Opendev build, religion works in a similar fashion to influence. Spreading your religion to your people and to other factions populations increases your religious points allowing you to upgrade your religion and attain new bonuses from the religious tree (chart thing). Your factions main religious building, along with various Wonders, certain civilization specific buildings, and unlockable buildings increase faith (like religious influence). The more faith you have, the less likely your people are to be converted, the more likely you are to convert others, and so forth. Having a high population with a high percentage of them as believers in your faith will increase the number of bonuses/upgrades you can receive from the religious chart/tree/scale thing. 

(If any of this is outdated info or I am missing something, could any other testers just correct the outdated info, cuz religion is probably the part of the game I currently pay attention to the least).