I read a very interesting interview with her, or possibly an article by her, on that aspect of the world building, but I can't now find it. There's an Islamic analogue that comes in later, but nothing like Christianity. (I have big questions about what Islam would look like without Christianity which the book has, understandably, no interest in answering, but that's just me!)

I think you're right about the characterisation being thin in places, but I still really enjoyed the series!