I like your backstory (subject to player approval blah blah blah - hopefully we can all take the social contract stuff as given).
For me the issue it raises is how to incorporate this cult into D&D play. There are a few d20 modules I know that you might look at if you're not familiar with, both for particular scenarios and more general ideas: Three Days to Kill (Penumbra); Maiden Voyage (Penumbra); What Evil Lurks (Necromancer). The ideas of sin and repentance loom larger in these modules, I think, than in typical D&D scenarios.
