From what I messed around with faith in the game, it seems to be a very redundant system, in certain ways. I mean, if you don't pick the "Religious Policies" that allow spending faith to buy things (like cathedrals and similar stuff), in a short time you don't have much to do with faith. It's like you generate faith to get policies to generate more faith. You can get some weak food/gold/culture policies, but they are very random. I constantly found myself with lots and lots of faith and without much to use it on very early in the game (maybe I'm missing something, though). Missionaries and Great Prophets after enhancing the religion seem kind of pointless.
Then, after a lot of time, you can buy Great People with faith. That's probably the most useful thing to spend faith on. Maybe the weird GPro mechanic of losing all faith when a GPRo is born (rather than use a treshhold) is used to avoid people getting at industrial era with thousands and thousands of faith points and buying thousands of great People.
I haven't played the game enough to make a good analysis of it. These are just my first impressions while playing.
Then, after a lot of time, you can buy Great People with faith. That's probably the most useful thing to spend faith on. Maybe the weird GPro mechanic of losing all faith when a GPRo is born (rather than use a treshhold) is used to avoid people getting at industrial era with thousands and thousands of faith points and buying thousands of great People.
I haven't played the game enough to make a good analysis of it. These are just my first impressions while playing.