@Tfmonkey Forgotten Realms' religions seem to mostly be some aspect of Christianity. Like Ilmater is the mercy and self-sacrificial masochism side. Lathander is the Sol Invictus side. Probably more, I don't have Forgotten Realms memorized.
A story about the dirty history of Christianity could be told in the form of an adventure, and it might look less like you told the story, and more like the authors intended it.
@RabbiMikeIsraetel the way I would do it, all religions are sun-based, but different religions have different "suns". Some are circular, some are stars, and some are crosses or "starburst" stars.
The moon is god's "wife", but she's not evil or anything. They are opposites, but not enemies, like Yin and Yang.
@Tfmonkey Shar might be the monastic side of Christianity. Loss of self to become a dark justiciar, "and it is no longer I who live, but it is Christ who lives in me" (Gal 2:20). Food for thought.