Need help/recommendations with a setting.

[Adalwolf]

Greetings from dark,rainy Greece! I am about to run an adventure set in the real-life Medieval Era and i was wondering if there is some kind of a splatbook or an online guide or something concerning the conversion of traditional Christianity (Heaven,Hell,God,Satan,Angels,Demons, everything) in D&D terms (cosmology,alignments,deities,domains etc). Any and all help will be appreciated and please feel free to express your own thoughts and ideas! Thanks in advance.

[Falcon X]

Oooh, one of my favorite topics. Your main sourcebooks should be Fiend Folio, Book of Exalted Deeds, Book of Vile Darkness, and Fiendish Codex 2.

Unfortunately, after 1st edition, and the Christian scare that D&D was evil, the gamemakers made a point of not letting D&D resemble Christian mythos. However, we can do our best to piece things together.

First off, the Christian Satan is Lawful Evil. You would do well to just use the sourcebook "Fiendish Codex 2" for most of your evil mythos. For Portfolios, I would use Death, Decay, Tyranny, Pact, and Evil domains, supplemented by the seven deadly sins (Wrath, Lust, Gluttony, Greed, Sloth, Pride, Envy)
The Christian God is Lawful Good. His defining traits, and thus portfolios are: Love, Justice, and Sovereignty (Fate). Supplement with Glory, Purification, Life, Good, and Exorcism domains.
- Asmodeus is the unquestioned lord of Hell, so he is most like our conception of Lucifer. However, an interesting bit of D&D lore (I forget which book I read it in) is that Asmodeus usurped his position by killing the former Lord of hell, whose name was Lucifer. Thus, the D&D mythos seems to be like a world built after ours, far in the future. Food for thought...
- Mephistophiles is the second ranking devil in D&D. He is actually BIG in Christian lore. You can read the most about him by reading Faustus by Marlowe or Goethe.
- The Fiend Folio has classes of tempters for fiends. USE THESE.
- Book of Exalted Deeds has Sanctified Spells and Fiendish Codex 2 has Investitures. Use these with your spellcasters.

The Christian God is a little different from D&D gods, so you may consider a few things:
- The Christian God (YHWH) has never been one to give His followers constant access to His power. Through history and lore, it's been shown as sporadically given based on the situation.
Thus, you would do well not to have the Cleric class at all. Substitute it with Crusader and a Wisdom-Based Factotum. Both of these classes work by being "inspired". It is much more like the Christian God to give his followers flashes of insight, guidance, and a rare miracle.
- For a Wisdom-based factotum, just change every ability that works off intelligence to wisdom. Then, have the small spellcasting progression be divine spells and represent those few divine miracles.

If you are willing to drastically change to something Christian-esque, Lord Ao and the Elder Elemental Evil (MM4) are the most similar gods to our Christian God and Satan.

[Kazudo]

A way I've seen "traditional christianity" done, without access to any proto-Judean sources (like the Lesser Key of Solomon, etc) was to never address YHVH directly, rather going with Patrons or Aspects for clerics and paladins.

If the Lesser Key of Solomon and any other proto-Judean works are permitted, then you're cooking with gas for the filler for the two halves of the "Pantheon".

The Neutral alignment would be tough, however. At this point you'd probably want to introduce Secularism, but I don't know how and don't feel comfortable discussing it too in-depth for fear of ruining my own net neutrality.

[Shining Wrath]

I'd start with Dante's Inferno, which is the model for the Nine Hells.
Then add Milton's Paradise Lost, which will give you a good working model for Satan's personality. Infernal grandeur et cetera.
Then pick up a history book about the Crusades.