Hi... Missionaries usually come from their particular church or denomination. An individual might decide to become a missionary after they feel the burden of responding to their religion's call for missionaries (for example, a Christian's call from the Bible can be found in Mark 16:15 "And he (Jesus) said unto them, Go ye into all the world, and preach the gospel to every creature"). They are then sent from and supported by their particular church or denomination to go into the world with the "mission" of sharing their faith and persuading others to join them in their faith. Often missionary work is accompanied by humanitarian work, such as caring for the sick (a medical missionary), teaching, running orphanages, etc.