The Missionary is a religious unit in Civilization VI. It can only be purchased with 
Faith in a city that has a majority religion and a Holy Site with a Shrine (or one of its replacements).
- Attributes:
- Action: Spread Religion to convert
Citizens to its religion. - Ability: Defend against (but not initiate) Theological Combat.
- Can ignore national borders.
- Action: Spread Religion to convert
Strategy
The Missionary is the earliest available religious unit and serves as your basic, cheapest tool to spread your religion to other cities. It may only be bought with Faith, and its cost increases for each unit built. Missionaries may easily fall prey to enemy Apostles when trying to convert foreign cities, as they have less 
Religious Strength than Apostles and can neither initiate Theological Combat nor gain promotions.
Civilopedia entry
Inevitably, in every religion, there are those who “hear the word” and then go out to spread it. Throughout history, the preachers and proselytizers have gone abroad, often at great risk – hence, all those martyrs and saints - to convert the non-believers into their faith, whichever faith it may be. That is not to say that missionaries don’t also do good works of a more practical sort: schools to teach, clinics to heal, social welfare programs to support, thrift shops to cover, and all the other benefits of civilization. Whether with a sword or a smile, the primary goal of the missionary is to share their faith and so gain new faithful. Whereas they once actually had to go out on missions to do so, now they can tele-evangelize, which is so much more safe and comfortable.