Most RPGs I've played have fictional polytheistic faiths or a sort of quasi-Christianity like the Chantry in Dragon Age. Are there any good RPG's with real Judaeo Christian religions or Norse gods? I've played some hack n slash games with Thor and Odin like Rune, and there a bunch of strategy games like Stronghold: Crusader that have Christianity and Islam, but I haven't played any RPGs as far as I recall..

Call of Cthulhu. :P

Unfortunately GOG doesn't have it.

Darklands for example

Vampire the Masquerade: Redemption is set in a "real world setting." Though religion is barely mentioned in passing there. It's also not medieval (after a short time) nor fantasy.

Shadow Hearts (PS2) kind of touches on Christianity, but again religion isn't a huge facet, despite one of the characters being a priest and another a demon monster hunter. Also not medieval or fantasy, though I do suppose it touches fantasy pretty close as well.

I think game companies (especially today) stay away from religion unless it's in a historical game because it's such a polarizing topic and always has been, and why would you want to alienate part of your potential money spending audience?

Darklands. It's set in medieval Germany, although a version of it where mythological beings exist, so Catholicism and the concept of saints is quite a big part of the gameplay and story.

Lionheart: Legacy of the Crusader may also have it, since it takes place in an alternate history version of our world. I'm not sure about it, though, as I haven't played it.

It's not quite what you wanted, but one of the Fallout: New Vegas DLCs all but says that some of the characters are Mormons.

Are the gods in Titan Quest based on Greek mythology or something?

There's also a Jason and the Argonauts game, but I think I heard it wasn't any good.

I heard it was good, but just good.

Then again, neither of us own it so who knows.

Darklands is the best example. The characters are all Catholic, the "magic system" is based on invoking saints for blessings, and even time in the game is told using church time (e.g., late afternoon is Vespers).

If I get around to playing New Vegas I'll be sure to check that out.

I want to play darklands but I have trouble with some of those older RPGs. I started playing RPGs in the later half of the 90's, so getting over interface issues can be a challenge. It sounds intriguing though and everybody says it's great.. I'll give it another go at some point.

Perhaps Unrest? Never played it, but it looks more social-realism than most RPG's.

As everyone said before, Darklands it's the best treatment of Middle-Age religion in a computer game (I checked every saint, even the most obscure ones, and their festivities are correct! Microprose was awesome.)
I understand the interface seems dated, but this is the very definition of a "good old game". That been said, you could try Legends of Eisenwald. I haven't played myself, but since it has the same setting as Darklands (late medieval Germany) with a modern graphic engine, perhaps it's what you're looking for.