Ideally we would have each empire and religion as a stand-alone thing, not depending on any other, so GMs could choose which they want to use for their SJ campaign setting, and ignore those they don't want.
Obviously this wont work, as all empires and countries have histories, and you can be sure part of the history will involve interactions with other countries. Religion is likely to have affected the history, even though by leaving out the evangelical religions of Christianity and Islam the effect of this might be reduced.
I'm not sure how we handle this. One way might be to think about the sort of detail that the GM will want, so as to be able to use the setting. For example, the first draft of the Mithras religion had detail that I don't remember seeing anywhere else, but we thought about keeping it to the sort of level as the information on each god in the PHB work better, or, some sort of intermediary level.
What sort of detail do we need about an empire or country? I suppose it depends what some of the background details are that a GM feels will be useful. As an absolute minimum things the GM will want to tell the players -
I added the last because one question tends to be "why are we adventurers?", at least in some gaming groups I've seen. One recent game involved a fear of press gangs a few months down the line! [grin]
Other players will want to know about neighbouring countries, and what the general relations are with them - friendly, neutral, hostile. Then there are questions about the history of the local area, and the (rough) history of the country as a whole, rarely more than a generation or so back, i.e. within living memory.
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