faith religion

Hi, I am creating a scenario that happens before Renaissance era (it has only Ancient, Classic, Medieval Eras). As we know, before industrial era the automatic faith purchase will be only Great Prophet and some Land units (if according beliefs has been chosen!). but I want at the end of...

I am playing on Prince difficulty and have managed to found my own religion. However, I found I was swarmed by missionaries from Japan and before I knew it my cities were all reverted back to my original pantheon. I am not sure what happened. I tried to purchase inquisitors and missionaries...