The most common reasons for early pantheons in my games are:
(1) Ruins
(2) Religious city states
Iirc a religious city state grants 6 faith when being discovered, so if the AI is lucky an has two around, it can found a pantheon quite quickly.
On a side note, does anyone know how pressure works?
Every city that has a religion applies pressure to any other city within range. Without religious modifiers, standard pressure is 6 and pressure range is - iirc - 10. Number of followers does NOT matter, only the dominant religion is important.
So, for example:
You have 3 cities, and two of them are taoistic. All three cities are within the pressure range of the city state Valetta. Your cities will thus apply 2 x 6 = 12 pressure onthe city state.
A but later, your 3rd city becomes religious. Now you will apply 3 x 6 = 18 pressure on the city state.
As you can see, religion can easily have snowballing effects if you found a religion early and/or have cities tightly packed together. A headstart of 10 turns with your religion can mean that it has "infected" a lot of cities before another religion had the chance to spread. That can also mean once that if a religion has spread far enough, it is almost impossible to counter-convert cities - only narrow passages or barely settled areas will slow the spreading down and allow you to create a "frontier".
A good way to test this is to try a "dense wide empire" strategy: Play maya on a pangae map, found 6 cities within min range around your capital, get pyramids and grab a religion. Your 7 cities will quickly convert and apply so much pressure tp anyone around you, that your religion can easily dominate the map and convert 2/3 to 3/4 of the map as the game progresses. I would, btw, recommend increased spreading range for this.
