It seems to me that a missionary has to accomplish some specific things:
- Sensitively expose sin and point people toward Christ.
- Help persuade people about the Gospel and help them understand the consequences of their choices.
- Help the church obey the Great Commission (Matthew 28:19-24).
- Teach people how to effectively relate to God (and not just know of Him).
So when I financially support a missionary or pray for them or serve and support them otherwise, I need to have these objectives in mind. They represent the body of Christ (of which I am part) - engaged in these specific objectives.