Civilization – Can any follower evangelize a religion

I believe you have lost the religious war, friend.

You are correct about Great Prophets; there are indeed a limited quantity of them. In my 6-player game on Prince difficulty, there were 4 Great Prophets. I believe you are also correct in saying they are used strictly for founding a Religion. I did not notice any other options for that unit during my game.

As you have mentioned, buying units with faith corresponds to the dominant religion in that city. If you still have citizens following your religion, there may be some small hope for the pressure to bubble and your religion to become dominant again, but that is unlikely at best. Cities spread pressure based on how many followers are in that city; the dominant religion will almost always be emitting the most pressure. If you chose traits that increase pressure in some way, your religion might resurface. Otherwise, I believe it has been lost to the waves of time.

DISCLAIMER: This game is a new release, and this evidence is almost all anecdotal from gameplay experience.

There are basically 2 sources of religious conversion in Civ:

1. Religious units doing their conversion action on your city
2. Religious pressure from other cities

Religious pressure is most likely what happened here. Religious pressure comes from nearby cities passively converting citizens of other cities to their religion. The closer the city the more pressure (I think the range is up to 10 tiles or so).

Note that having faith buildings such as holy sites does not inherently make your citizens religious.