I came across this statement from John Piper on missionaries who go where the gospel is not preached. Well said.
My aim here is to celebrate the immeasurably important work of missionaries. There is nothing like it in the world. Nothing can replace it. Oh, what a rare band—what a rare breed—of human beings are the pioneer missionaries who say with the apostle Paul, ‘I make it my ambition to preach the gospel, not where Christ has already been named, lest I build on someone else’s foundation, but as it is written, “Those who have never been told of him will see, and those who have never heard will understand”” (Rom.15:20-21). The One whom the nations have never heard of, the One they will see when we tell them, is Jesus. In spite of all their sinfulness and ordinariness, there are no people more to be admired and encouraged than the missionaries who share this holy ambition (John Piper, Jesus: The Only Way to God: Must You Hear the Gospel to be Saved? [Grand Rapids, Mich.: Baker Books, 2010], 12-13).