A Europe reverted to feudalism is something I've considered, as are a neo-Catholic/Roman Empire- probably ruled by human hating mutants and/or cyborgs- and, since you need a foil, possibly a neo-Orthodox/Byzantine Empire; likely ruled by humans. But doesn't that sound a bit cliched?
Not only is it a little cliched, it is rather anti-Christian in tone and it's a good job that such groups don't get aggressive about misrepresentations of thier views. I would advise sticking to neo-Roman or neo-Byzantine, and not involving pseudo-representations of faith groups.
Question #1: How in the name of the Holy Tabernacle of the Sci-Fi Mutant Princess was that bare bones outline of an thumbnail idea for two opposing power groups extrapolated as being "anti-Christian"?
Most post-apocalypse settings tend to be extrapolations of modern society, taken to the ludicrous extreme, and projected forward through the prism of whatever cataclysm brought the "end" or "fall" or whatever. For the most part I have been trying to keep away from overtly obvious representations of "faith groups" (the pseudo-Mormon Empire not withstanding, that's just too cool an idea) and Iridium does have a "piety" attribute.
Question #2: Do you all really feel using piety and having psuedo-religious groups in MD is going to be a major problem?
I can always do away with piety, granted Bill might say I don't need much of an excuse to do that
, but I'd like to keep some of the wild and wacky wahoo pseudo-religious groups.