A few bags of stuff to work out:
1) "Immoral" actions by PC's. A LOT of this is going to be VERY subjective, depending upon the specific version of Christianity, and the specific beliefs of the players themselves. A Christian Player's Handbook wouldn't nessecarily, say, dismiss with Forgery or Bluff. But they would let it be known that Good Christians don't use thse, and the use of these skills would require repentance, if they are used (humans ain't perfect -- sometimes, for good ends, people must use underhanded deeds). And, at the same time, it's still open to a bit of theological debate...is depicting God or Jesus in artwork considered an evil act? What about eating pork? A *lot* of these will vary depending upon what "flavor" of Christianity you're using...even what style of world you're using. Thus, I think omitting things from a PHB that "good Christians" wouldn't use would be rather silly and hindering -- because what "good Christians" would do would vary by campaign, just as it varies as to what good heroes could do, or what good Paladins can do.
2) Law vs. Chaos and Goodness.
I think those who say that that worshipers of Christianity would have to be Lawfu haven't pondered it quite enough.
I mean, again, it largely depends on the aspect of Christianity you're using to determine the world's morality...certainly, a Catholic might draw power from the hierarchy of the church, but a Protestant would probably balk at that order, as would even good Christians in the middle ages (the Divine Comedy was written by an outcast who absolutely loathed the Pope...not exactly a Lawful person, IMHO). You don't need to remove law or chaos at all, I think...though, of course, Good and Evil are MUCH more important.
3) Arcane Magic.
This doesn't *have* to be evil, if you're working with Christian themes. Again, it's campaign-dependant...wizards are a lot like fantasy scientists, and many scientists are and were devout Christians. In the middle ages, the christian monks and priests were the most educated people around. The stipulation would be that it would have to all relate to how God intended Man to use his creation. If arcane magic were the purview of Wizards, it could easily become a cerebral thing -- God gave Man the power of Magic so Man could defend himself against the forces of evil. And, like any knowledge, it can be used with Man's free will for EVIL!
The above reasons are largely why I consider a campaign setting to be the most important thing.
So, for a few "campaign seeds:"
* In a campaign set in or around or right after the time of Jesus, you have most true believers being Jews following James, Jesus's brother. You have probably many "gods," all capable of imparting divine magic to their followers. Of these, Yaweh, the god the True Beleivers follow, is the highest and most supreme and only *true* god...the rest are demons, poseurs, etc. They still have divine powers -- the reason why is as dark as the reason that Yaweh allows evil to exist at all. There are a lot of prophets and seers running around, proclaiming gifts of the divine. The Sorcerer class can be used to represent these asthetics (or Psion), and his powers are largely indistinguishable from divine magic, and may be mistaken as such...espcialy if they have a lot of divination. The controllers of the time are both the pagan Romans, who rule the region (using mostly military might), and the Jewish sages, using power from Yaweh (though not in the right way, of course). Jewish sages may also be able to access the learned arcane magic -- the Wizard class.
* In a campaign set closer to the Roman age, you see a division occuring. Either you're playing as the Good Guys against the Big Bad Empire, right at a hotbed of demons and devils who grant spells to the machinery that puts you down and feeds you to lions, or you're playing after the conversion (which, IMHO, is a LOT less interesting
). There aren't a whole lot of changes from above, except that Wizards are now more likely to be wicked (but educated) Romans, and Sorcerers could be your ace in the hole. Roguish activities are the survival method of your faith. You worship in the catacombs, skulking through the shadows. You are accused of doing evils that others are truly responsible for -- and must fight against those evils that arise within your own ranks (for no human is imune to corruption).
* In a campaign set more in the middle ages, Wizardry is the learned Man's weapon, but also a corrupting power -- it certainly can be used to oppose the Church. But, of course, the Church is turning evil itself -- high - level beurocrats are using people's faith for good and evil both, and the combined worldly and spiritual might is difficult to fight against. It wouldn't be out of dogma for you to play the "good guys," put down by the current state of evil within the church, and trying to right a greivous wrong in the affairs of the religion. At any rate, the power of the evil spirits is on the decline -- no longer do demons and devils power the ruling forces of the world. For the most part, they are Good, Honest individuals. Of course, power is hard to give up, even more so for a wicked creature. The mere humans put in the positions of leadership certainly aren't immune to the corrupting forces of evil that greedily press around them, hungry for the power hey once had. The other stronghold of evil is in areas where the light of Christianity has not been braught. Some worship God through different names, but most are, in fact, being led astray by the corrupting power of evil throughout the world. Sorcerers are no longer the chosen ones of God, usually -- normally, they are those with the blood of demons in them, working evil for their people, as the demons and devils, weakened, loose the power to grant their followers spells as normal divine magic. This lasts right up to and beyond colonial times...
*In a Modern campaign...it is VERY flexible. I'd avoid using the supernatural much at all, and focus on the trials and tribulations of being a human today -- religion is bound to get involved either way, and, today, none of us can tell by magic who's good and who's evil. OTOH, if you *want* to heavily drive the supernatural, I'd say say bring it up with your players...there's a million an one really cool ideas if you're willing to suspend belief and "pretend" that one belief or another is entirely accurate, but that is definately shakey ground for anyone. Personally, I'd be willing to play in a campaign were all religions were equally valid and powerful, and divine magic results more from your belief than from the divinities themselves, which are unapproachable beings, much as they are in Real Life. But that's becaue I'm generally comfortable having things in the Game be a lot different from Reality. But this is definately an issue that should be dealt with on an individual basis -- no guidebook can be made, because too many variations exist.
