I think the cover I saw was a sandy/sandstone color, with a red feather. I'm not sure if it was supposed to be a dystopian future, or a distant past.The plot revolves around a sacred cave that two opposing cultures claim as the site of their God/gods. A girl from a city that doesn't believe in any deity sends her as an emissary on the trip to see who is right. The atheist girl has a romance with the polytheistic boy. The monotheistic girl pretends to be too pure for that, but watches them out the window.When they finally reach the cave, the three kids (late adolescents?) each have a spiritual experience, and the two that already believed in God/gods take it as confirmation that their belief is correct, and their people use it as an excuse to slaughter each other. I think the polytheistic boy and monotheistic girl die, and the atheist girl hides in the sacred cave until the fighting is over. She goes back to her city, and finds that no one is willing to believe her experience. I do not know the author, or if there were sequels, but I read it in the adult section of the library in 2005(ish).The atheist girl lives in a city like a monastery, the polytheist boy is described as being 'savage' and his people are tribal, and the monotheist girl lives in an almost industrial city, with more advanced weaponry and armor (I think everything is built of stone?).
In order to stop a religious war, Kyra and her companions, Lillen Statual and Ignis Orlendawen Eli, embark on a quest to a forbidden cave to seek the ultimate truth about religion.