If you're going to have 5 flavors of Christianity, how many flavors of Islam (just Shia/Sunni or Wahabbist, Sufi, ....), hinduisum, and so forth?
And why just the European Christians? No Coptic christians or Monophysites?
Getting into this kind of detail is a mistake I think.
[If you did pick an Orthodox Christian Holy city it would be Constantinople.]
I think this falls under the the same issue as city-states generally, which is that there's no real reason not to just have all of them. Scientology, Wicca, Kim Il-Sungism, Asatru, World of Warcraft, I'm sure there's millions we could think of. We certainly came to bat in the missing city states thread.
Knowing what we currently know of Civ 5 without having actually played it, here's how I'd run this in a mod:
Step 1: Script religions into the Civs
- Religions would be empirewide, not by city.
- There are no religion-specific buildings* or units.
- Civs would ignore Religions for Civ-to-Civ diplomacy, as City State diplomacy should play well enough.
- Civs can switch religions from start of game with no direct penalty*, or choose not to have a religion.
Step 2: Create and script "Religious" City States (as opposed to MAritime, Cultural, or Militaristic).
- Provide Happiness and a modicum of Culture.
- Are ascribed a relevant religion.
- Can ally with ALL Civs that share their religion, are never worse than neutral with Civs that share their religion.
- Are never happier than neutral with Civs that are another religion, and never above friendly with Civs that have no religion.
- The first Civ to ally with a religious city-state receives the "Holy City" wonder for that religion in a random city. This provides happiness, culture, and gold. If the Civ changes religion or abandons religion, it loses some or all of this bonus.
- The City State only declares war if ALL allies have declared war, and never on a Civ that shares its religion.
- Other city states do not ascribe to a specific religion and play as normal.
Step 3: Script events, quests, and what not for the Religious city-states to pursue
Step 4: Script Religious city-states into the world creation
- Balance so that ~25% of city states are religious in any given game. Say 3 to 4 on a standard map.
- To allow for lots of variety, have City States that otherwise would be parts of Civs, only pop if the related Civ isn't playing - So Boston(Puritans) and Salt Lake City wouldn't show if Americans were in play.
Step 5: Adjust SocPols as needed (Hard to tell what needs to be tweaked without having played the game)
I think this would provide a realistic and interesting enhancement to gameplay, without bogging like religions can tend to in 4. The existing City States mechanics should carry it the rest of the way.
Oh, hey, Port au Prince could host Voudon, and Kingston Rastafari.