Lets talk about God. I am actually growing more in favor of allowing Christians into the setting. I think that a sidebar explanation that for game mechanic purposes only Christianity is treated the same as other religions, many of them historical or mythological and no comparisons or allusions are implied. Then if people still get uptight fuck them plus - free publicity!
From a game standpoint Christianity would be the same as any other religion and/or divine power that provides the source of priest magic of whatever pantheon. Given the different Christians we can bring it even adds more to the political intrigue. I don't think we need 100 million Christians running around, but some couldn't hurt I don't think, and, as you point out, it makes adding certain other cultures to the party easier.