Their UA nets them +1 faith in any city with an adjacent unimproved forest tile, +2 in a city with three unimprover forest tiles. Because their start bias places them near forests, youll always start producing faith from turn 1. Youll be founding your pantheon within 5 to 10 turns after startup (on quick game speed, short attention span ftw!), your official religion a littel bit later, and your full on world religion before the AD rolls around. And by that time, there still wont be any other religions even founded yet. Stack that on top of their UU, the pictish warrior, which replaces the spearmen and generates faith equal to half the strength of any unit it kills, the celts play the best religion game in all of Civ V. The Celts, definitely, and Ethiopia as a second. The Celts can develop faith without needing buildings, and depending on the terrain, they can develop a lot of it.
In the first part of the game, they're a religion powerhouse, and they can start a pantheon in a minimum of 5 turns into the game. Their unique unit also gains faith for every kill, making the even faster at starting a religion.Ethiopia's unique building (can't remember what it's called off the top of my head) replaces the monument and adds faith to it. That's really the only reason they're good with faith, they get an early start on faith buildings while building their culture.Byzantines are another interesting choice because of their bonus belief. It only comes into play later in the game though. Depends what you mean by best.
Do you mean will get the first Pantheon? OR Will found the first Religion? Because these are absolutely not the same thing. While Ethiopia or the Celts will probably get first Pantheon, they can easily be overtaken to first Religion by someone who gets Stonehenge, or picks a Pantheon that gives them a massive inherent faith boost etc.
More on this below.Secondly, the Celts are only the best if you are playing with Map Bias ON. Then the game will start the Celts adjacent to 3 Forests.
Choral Music is really good. 6 culture if you have a shrine and Temple. Feed the world is the food equivalent; great for Russia on the tundra.
Same deal with Spain.Map Bias will give them a natural wonder on their doorstep. If you are playing the 'purer' game with a truly random map, Ethiopia is the best.However as I said, that is only better from a starting a Pantheon view. Starting a Religion is a different matter. Example, you might be 4th or 5th to get a Pantheon, but if your first few cities are sitting on a lot of desert and you take the Desert-based Pantheon (forget its name sorry), you will almost certainly get first Religion. I did this on my most recent game, and my Faith was sitting at +20 by the time I got my 3rd city up. +30 shortly after once the population grew a bit. For nothing, basically.
I got first Religion miles before all the others, and I had built only a single Shrine in total.