Civilization – Who wins if I spread other players 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.

Their actions in order:

• Evangelize Belief - This allows you to add in extra Tenets to your Religion (on top of your Pantheon and Religion bonuses)
• Launch Inquisition - Allows Inquisitors to be recruited, which can remove the presence of other religions.
• Spread Religion - Like Missionaries, Apostles can spread religion. They have a chance for promotions that make them better at it, however.

Apostles (or either of the other two religious units) can only recover HP on (or adjacent to) Holy Sites of their Religion. Move the unit onto, or next to, a Holy Site and sleep / skip turn until they're back to full health.