How to found and spread your own religion
Later during the Exploration Age, you can found your own religion. This is done by studying the Piety civic and then building a Temple in your settlements. When you found your own religion, you may also select one Reliquary Belief and one Founder Belief.
Likewise, take note that you can train a Missionary in any settlement that has a Temple. Missionary units have “Spread Religion” charges. All you need to do is move them to another settlement to convert that place to your religion.
Note 1: You may also recruit Missionaries from settlements without any religion or those that another civ converted to their religion. The only important thing is that the settlement has a temple. For example, you can forward-settle a town close to another civ, buy a Temple using gold, and then buy Missionaries so they can start converting your opponent’s territories.
Note 2: Converting a settlement’s population depends on the tile where you used the “Spread Religion” action. A good example is how large cities have separate urban and rural population. If you want to fully convert a city to your cause, then you need to use the action on (a) an urbanized district, and (b) a rural district, such as a mine or a farm.