Pressure has nothing to do with numbers of followers or faith generated in a city or the existence of religious buildings (exception is the Grand Temple). Each city with a majority religion exports 6 pressure (at standard speed -- 9 on quick, 4 on epic and 2 on marathon) that is "felt" or "experienced" by every other city within 10 tiles. Pressure experienced by a city is simply a function of the number of majority religion cities within 10 tiles of that city. When you mouse hover over a city banner, the pressure numbers you see are what that city is experiencing, not what it is exporting (just exports 6 pressure). So, you will most commonly see pressures that are multiples of 6 (e.g., 42 pressure means there are 7 majority religion cities in a 10-tile range).
With Itinerant Preachers, the pressure range extends to 13 tiles. With Religious Texts, the per city pressure increases to 8 (and to 10 after Printing Press -- so those pressures will be multiples of 8 initially and rise to multiples of 10 thereafter).
Holy Cities are unique. They get 5x normal city pressure (e.g., 30 pressure at standard speed) that is felt only internally -- not exported to other cities. So, a Holy city alone will have 30 pressure, while one with 2 nearby majority religion cities will show 42 pressure (30 + 12).
A Holy City with the Grand Temple will export twice the normal per-city pressure (i.e., 12 instead of 6).
