Cities with a dominant Religion will apply religious pressure on other cities within 10 tiles, slowly converting the citizens of those cities to that Religion. If more than half the citizens in the city are Followers of a particular Religion and that Religion has the majority of Followers in that city, that Religion will become predominant in that city. This will convert some of the citizens of that city to Followers of your Religion. To do so, move the unit adjacent to a City Center-or inside it, if the city is yours-and use the Spread Religion action. Religious units' primary purpose is to spread their Religion to other cities. Religious units may be purchased through your city with Faith and will adopt the majority Religion of the city in which they are built.
There are four types of Religious units: Missionaries, Apostles, Gurus, and Inquisitors.