I like the mechanics how each cultist preach from their own holy book, but there are few elements that doesn't sit quite right with me. First is that cults have only one message instead of whole belief system (what is sin, how to repent and what you get afterwards). Second thing that I don't like is that there is only one GM. This could be changed to GMless system where each prophet takes turns representing the problem then other prophets try to solve it according to their holy book. Also I feel that problem should be presented before cultist turn.
Overall the setting is fine, mechanics are great but could use little fine tuning.