I can see a "pressure" value on cities which seems to indicate how powerful is the conversion to a religion in a city. But how can I translate that value to some meaning? What does "+4 pressure" imply? What's the difference between +4 and +20? Can I tell how long it will be until a religion converts a citizens by looking at the pressure value?
So far it seems to be that the pressure is the percent chance of converting a citizen on any given turn. I will edit this answer if I can find better evidence. It definitely appears to be a flat scale, 12 pressure converts twice as fast as 6 pressure. I had one city with 12 christian pressure and 6 buddhist, and it was 2/3 christian and 1/3 buddhist after a while.