Here is what I have been able to figure out:
Playing at the Byzantines on Epic without any religious traits that increase the spread of religion, it seems that your holy city gives off 20 (maybe 12?) pressure. From what I can tell there is a point limit, much like a great person limit, before a citizen is converted to the religion. It took a while for my capital/holy city to become 100% Christian but it did happen. I assume you would have to have a ridiculously fast growing capital/holy city for your entire population to not be eventually reach 100% conversion. Mind you this is with no other religions competing against it.
As for spreading, my second city which is less than 10 tiles away started with a pressure of 4 for my religion and it took a LONG time for it to become the majority religion (it still isn't 100%). Interestingly enough, I made a 3rd city <10 tiles away from this one and until a city has a majority of religion, it won't spread to other cities so I was in this situation:: 20 (12? can't remember) pressure on the capital, 4 on the 2nd, 0 on the 3rd. Luckily I had a lot of extra faith (Mt. Sinai ftw!) and bought a missionary. I had the wonder that gives missionaries 3 uses so I used this missionary (either 1 or 2 times... I can't remember, then I sent him to convert a CS) on my 3rd city. This is where it got interesting for me. The city was a majority Christian, but it has no pressure on it. However it SEEMS like once you reach a majority and there is no pressure from another religion, the population scales. If you population is 5 Christian out of 9, it seems to go to 7 Christian out of 12 (for example) as the city grows, even without pressure.
I say that because after doing this, my 2nd city now had 8 pressure and it finally became a majority of Christian. This in turn also gave my 3rd city 4 pressure and so on.
As for foreign religions:
I then used another missionary to go into my neighbor Rome and to convert one of his cities (city 2). If you made a line of cities it would look like my capital ---- 2nd city ---- 3rd city ---- roman city1 ---- roman city2. Therefore I had my 3rd city and his 2nd city giving pressure to his 1st city and some others. I did this for a while, no need for details but eventually he founded his own religion and things got intense.
In Rome, his holy city, he also has a 20 (12?) influence and it began to wipe out my religion in his cities (12>4 or 8 obviously). I still had some followers in his cities, but they were no longer the majority and eventually I think 1 city had all of my followers wiped out. To counter this I decided I needed a great prophet (one of my religious traits was that prophets were cheaper and more effective). I sent the prophet to Rome to counter his religion.
I'm hoping someone can clear this next part up because I still don't understand it.
I used the prophet and it was so effective it converted Rome to a majority of my religion. Unfortunately, the next turn, it was like it never happened. Christianity was back to 0 with no pressure and his religion was back in full swing. Is this intended, is it a bug? After trying a couple of more times, I reloaded and tried a different strategy. I went to other cities in his empire and converted them. Long story short, he had multiple cities (including my 3rd city) putting pressure on his empire so some of his cities had 12 pressure for my religion but the max he had for his religion was 4 (only 1 city, the holy city). I'm slowly taking over his empire and gaining all sorts of benefits but I still don't know what is up with the holy city deal.
On a random note, civs do NOT like when you counter their religions! Rome and I were best buds until I started to spread my religion and counter his. He hasn't declared war on me, but he denounced me and doesn't trade with me nearly as much now.
Long post, but hopefully that helps some of you and maybe someone else can come along and explain it even better
