How should churches evaluate the success of missionaries they support?

That’s a tough question to answer because competing principles are at play.

1. Supernatural fruitfulness can’t always be measured. Remember, missions work is accomplished “by faith, not by sight” (2 Cor. 5:7).
2. One of our most important criteria should be whether or not a person is faithfully preaching the Word and living a life of conformity to the Word.
3. Nonetheless, the Lord gives different gifts to different members of the body. This means that it’s possible for a person to labor faithfully in a work for which they are not gifted, which may prove discouraging to all parties over time. Through ongoing relationships with such missionaries, a church can get a sense of their gifts and whether their work is a good fit for their gifts and personality.
4. Now, a church should be willing to be patient and give time for the Lord to work according to his clock, which will often involve more time than a business waiting for a return on its investment. The first American missionary, Adoniram Judson, didn’t see a single convert for seven years, but ultimately his work contributed to the conversions of thousands.
5. Still, we want to be good stewards of the Lord’s resources, both in terms of the money he’s given, as well as in terms of the lives of the missionaries being spent.
6. The bottom line: all these factors need to be considered on a case by case basis.