John Piper writes in his biography on Adoniram Judson,
Patrick Johnstone says in Operation World that only in the 1990s did we get a reasonably complete listing of the world’s peoples. For the first time we can see clearly what is left to be done. There are about 12,000 ethnolinguistic peoples in the world. About 3,500 of these have, on average, 1.2% Christian populations—about 20 million of the 1.7 billion people, using the broadest, nominal definition of Christian. Most of these least reached 3,500 peoples are in the 10/40 window and are religiously unsympathetic to Christian missions. That means that that we must go to these peoples with the gospel, and it will be dangerous and costly. Some of us and some of our children will be killed (John Piper, Adoniram Judson: How Few There Are Who Die So Hard!, 6-7).
The need is mind-boggling in scope. So the question remains, “who will go?”