Game Info
Religious unit. May only be obtained by purchasing with 
Faith Faith in a city with a majority religion.
- Common traits:
- Limited visibility
- Abilities:
- May enter rival territory
- Does not incur trespassing penalties in non-friendly City States.
- Spread Religion (2 uses, 3 if built in the city with Great Mosque of Djenne)
- Unwelcome Evangelist - loses Conversion power while in a territory your nation isn't allowed to enter (-250 per turn; if power reaches 0 the unit dies)
Strategy
The Missionary is the basic way to spread your religion beyond the automatic city spread function. Once adjacent to a city, he can spread his religion to it (expending the Missionary after the second use). The effect of the action, or rather, the number of citizens that convert to the Missionary's religion, may vary according to many factors.
If Missionaries enter the territory of another civilization without an Open Borders treaty, they will lose 25% Religious Strength - usually 250 Religious Strength - per turn to "attrition." When their Religious Strength reaches zero, they are removed from play.
Note that each Missionary represents the religion of the city where he was born (purchased)! This can be useful in some cases. For example, you might temporarily convert a city to another religion allowing you to build Monasteries or other similar buildings. Be careful not to spread the wrong religion inadvertently. You can always check the religion of a Prophet or Missionary simply by hovering over it.
If produced in a city containing the Great Mosque of Djenne wonder, the Missionary will be able to Spread Religion an additional time before expiring. Also, if his religion has the Interfaith Dialogue Belief, and the target city has a different religion, the action will also yield a number of 
Science Science points.
Civilopedia entry
Missionaries are members of a religious group or sect sent abroad to spread the message of their chosen religion. While their primary goal is to share their religious views and hopefully gain new followers, missionaries typically provide other beneficial services to the areas they visit. By setting up clinics, schools, and other social welfare programs, missionaries help those in need while also serving in their evangelical role.