Concentrating on the game part---
I guess the question is really how much you want this issue to be central to the game. After all, that is what mechanics are for, representing the level of importance of an idea in play. That's why, in D&D, you have a huge list of spells, magic items, and monsters: the fantasy aspect is the most important part. okay, so I am sliding off topic...
Anyway, if the true role of God and Satan are important to your game, you are going to have to spend some serious time determining exactly what effects that has from a mechanical perspective. there are lots of good ideas here and I won't be so bold as to tell you which ones you should use. however, if you merely like the Crusades as a backdrop for the campaign -- a campaign that will still have a D&D feel to it -- then it is not so important. Give Christian Clerics the "good" daminas and give Satanic cultists the "evil domains" and be done with it. Domain powers represent the special powers granted by said deities. If you don't have to explain it, don't. Will your players really care whether Satan is a god or a solar with class levels?
There is, of course, a middle ground, where the crusades are a back drop for a campaign revolving around theology. Are the characters supposed to be elarning something no one knows (God and Satan are the same entity; angels and demons are different entities entirely and not the product of rebellion; etc...)? Are they supposed to be stamping out the heretics? Are they Raiding for the Lost Ark (best D&D campaign ever, btw)? In thse situations, I would actually remove divine classes entirely and have sorcerers be the forces of darkness and paladins be the forces of good (maybe even rangers, since I believe there were saints of the wilderness). That way, the mighty divine stuff can be plot points instead of crunchy bits 9as would seem appropriate for the above mentioned campaigns).
I guess the question is really how much you want this issue to be central to the game. After all, that is what mechanics are for, representing the level of importance of an idea in play. That's why, in D&D, you have a huge list of spells, magic items, and monsters: the fantasy aspect is the most important part. okay, so I am sliding off topic...
Anyway, if the true role of God and Satan are important to your game, you are going to have to spend some serious time determining exactly what effects that has from a mechanical perspective. there are lots of good ideas here and I won't be so bold as to tell you which ones you should use. however, if you merely like the Crusades as a backdrop for the campaign -- a campaign that will still have a D&D feel to it -- then it is not so important. Give Christian Clerics the "good" daminas and give Satanic cultists the "evil domains" and be done with it. Domain powers represent the special powers granted by said deities. If you don't have to explain it, don't. Will your players really care whether Satan is a god or a solar with class levels?
There is, of course, a middle ground, where the crusades are a back drop for a campaign revolving around theology. Are the characters supposed to be elarning something no one knows (God and Satan are the same entity; angels and demons are different entities entirely and not the product of rebellion; etc...)? Are they supposed to be stamping out the heretics? Are they Raiding for the Lost Ark (best D&D campaign ever, btw)? In thse situations, I would actually remove divine classes entirely and have sorcerers be the forces of darkness and paladins be the forces of good (maybe even rangers, since I believe there were saints of the wilderness). That way, the mighty divine stuff can be plot points instead of crunchy bits 9as would seem appropriate for the above mentioned campaigns).