Hi Funak,
tu_97 has compiled a nice write-up about religion here:
http://forums.civfanatics.com/showthread.php?p=14321211
We are now discussing the active religion spreading mechanics. Do you know if missionaries can convert only non-believers or if they can convert believers of rival religions in foreign cities to some extent as well? I think they can, but I do not know the numbers or it exactly works.
Okay, this is the deal with Religious pressure from what I've gathered, it got very little to do with actual followers, and saying things like a missionary will convert X followers is very misleading.
Unless I'm mistaken here, which is quite possible as I've never bothered doing any fact-checking on it, I only know that the ingame mechanics works out, cities collect religious pressure like a huge tank with no limit. Pressure from other religions doesn't overwrite existing pressure it just builds up along with it, the followers are then allocated according to the percentages of the total pressure in the tank that each religion has provided. For example if a 20 pop city have been influenced by 15 missionary uses from Christianity(15k pressure) and 5 missionary uses from Islam (5k pressure) the total pool would be 20k with Christianity holding 75% of that and Islam 25%, which would result in 15 followers of Christianity and 5 followers of Islam.
On top of missionaries, nearby cities adds pressure to the pool every turn (which I believe is the same value as the number you'll see if you mouseover the religious symbol on the city) although it might be multiplied with something for all I know as with 5 pressure per turn it would take 200 turns to build up as much as one use of a missionary.
Besides this there seems to be a soft-cap of pool pressure that you need to reach for every follower (I have no idea of the number here), which is why cities without competing religions don't always have 100% followers of the religion it's following.
As you probably see, with every used missionary the next one is going to bring less impact, if you already have a pool of 20k another 1k is only a 5% increase while if the pool is just 1k another 1k is a 100% increase. This means as the game goes on missionaries start feeling a lot less useful, you can still use them to tip the balance slightly, but that's all there is to them. Inquisitors and Great prophets on the other hand removes all foreign pressure from the pool completely, which gives you gigantic swings later on. Other than that, the only thing I know of that removes existing pool-pressure is the Reformation belief 'One World, One Religion' which makes missionaries remove foreign pressure when used, although I don't know if it's a numerical or a percentage value that gets removed.