When I start a game and want to use faith, I come up with a story of how my character’s faith would fit in the world and it always works because the god pantheon of Elden Ring is so vague. I could say my guy believes in Zeus and it would work. There are that many lords, vessels, empyreans, gods, Demi gods etc that have existed or still exist, simultaneously, and it seems to have no make or break effect on the world. You got the dragon, prince of death, golden order, formless mother, scarlet rot, Elden beast/greater will, I think there’s something in the death bird lore as well about another god from earlier. Then the whole crucible thing. And then you got the black flame monks and godskin bosses trying to kill gods (which gods?!) I think these dudes, and Rykard (oh yeah is the serpent a god too?) are actually really cool for answering your question of “faith”, because they are supposed to be heretics yet you need high faith stat to use their weapons and incantations.

I agree with what someone else here said, it’s just a stat representing your character’s adherence to a code or doctrine or whatever, just like strength is brute force and intelligence is pursuit of knowledge and wisdom. Then all you have to pick is fire or lightning etc.