I'm not saying that +5 faith is 100% perfect. Maybe it's too much, maybe it isn't. The problem is that the argument of "+4 yields for the low low price of 3 citizens!" isn't really taking in the grand picture. Let me quick say: yes, it is better than the God-King 4-for-6 that others have access to. Moving on.
Look at Desert Folklore. +2 Faith/+1 Food/+1 Gold from improved desert tiles. Your city starts at some desert territory. Your next city or two will be near some desert terrain as well. A decent city will have what, some stone, incense, some wheat fields, maybe iron hills, oil, heck let me list them all according to the Civilopedia:
- Iron
- Oil
- Aluminium
- Uranium
- Sheep
- Stone
- Gold
- Silver
- Gems
- Marble
- Cotton
- Incense
- Copper
- Salt
- Amber
- Jade
- Lapis
Your starter city is going to have 6-10 of this stuff easy. The next two have 5ish too. Yes, you have to improve them, but you were going to do that anyway. So you maybe buy a worker with some early gold or make one, and start improving. Let's be generous and say that you manage to hookup only a mere 5 of them by the time you are 10 pop.
10 pop = 3 "Celtic 4-yield" (3/3/3/3 aka 12). 5 Desert Folklore = 2/1/1 * 5 = 10/5/5 (aka 20). Sure, it doesn't include science, but its almost DOUBLE what the Celts have. And remember, the Celts
can't pick standard beliefs at all. But wait, math isn't done!
You have 10 pop. Let's say you got your pantheon at 3 Pop. You now have... two followers. What? Yes, you don't get everyone all at once. When you hit 6 Pop, you will for a time only have 4 followers before it will jump up to 5. When you hit 10 pop, you'll only have 7-8 followers. No matter how fast your Pop grows, you only gain followers at a standard rate. Remember you are Celtic, so you are relying on passive spread, so you can't get a mega 10/10 citizen boost from a Missionary.
The above scenario is quite realistic. Your 2nd city, when it is pop 5, can easily be working 5 improved desert resources. Once again, that is 10/5/5 = 20 yield. That same city with the Celtic-4-Yield will, at best, have 5 followers, aka, 1/1/1/1 yield. Four versus Twenty. Which do you prefer?
Later in the game, yes, the Celtic-4-Yield can be powerful. Remember though that it is their UA, so it's supposed to be better than standard. But you have to keep in mind that by the time that it becomes amazing +5 faith is a pathetic drop in the bucket. You have any other Civ with a religion? Enjoy making 50-200/FPT by the Renaissance. 5 Faith? Big whoop.
If you have a religion as the Celts, +5 faith is basically nothing (5 faith will buy you that 1,000 faith GP in only 200 turns! What a bargin!). If you don't have a religion as the Celts, then that +5 faith doesn't really do much anyway. You won't have any special buildings, you won't get the religion founder buildings, no pagodas, you won't have unit-faith-buy. All you'll have is access to buying GP with faith from full policy trees. In either case, +5 amounts to nothing long term, and short term, half the regular pantheons literally blow it out of the water and leave it crying.
Yes, my example used Desert Folklore, one of the "more OP" beliefs. The point is though, that they benefit from the +5 faith
without it being ungodly powerful; I believe it is overall balanced and useful, if not necessary.