This past summer, from evangelical churches nationwide, more than one million of the faithful departed for the mission field, taking up Jesus' "Great Commission" to "go and make disciples of all nations." The churchgoers hoped to convert souls, establish churches and meet other human needs. But they did not intend to serve for years or whole lifetimes, like such pioneers as Jim Elliott, who was killed in Ecuador in 1956 evangelizing to native people; or Hudson Taylor, the founder of the China Inland Mission; or even the awful fictional caricatures of African missionaries in Barbara Kingsolver's novel "The Poisonwood Bible."...