There are a few threads that go in to detail, but a rough version is a follows.
The pressure on a city from any one religion is usually 6x the number of cities within a 10 tile radius which have this religion as a majority religion. (religious texts ups this to 10x, and itinerant preachers to a 13 tile radius). There is also a +30 influence on any holy city.
So your city had 30 pressure cos you only had 1 city with a majority and it was your holy city. The opponent had 60 pressure in theirs either because there were 10 nearby cities following that religion, or because there were 5 nearby cities of that religion and it was their holy city.
How influence affects followersin the city is a little more complicated.
From the sounds of it, one thing to understand is that the religious pressure in a city affects the numbers of followers in that city only, and doesn't directly affect other cities. It's only number of nearby cities with a majority in that religion. Of course, to get a majority in a city.....
Hope this clears some things up.
