"Can the West be converted?" That question gripped longtime missionary Lesslie Newbigin when he moved back to his British homeland and saw its post-Christian condition. Likewise, renowned author C. S. Lewis viewed Europe and North America as a mission field. Lewis said that Christians in the West needed to learn the "mental habits... of their unbelieving fellow countrymen," much as overseas missionaries had to learn the language and traditions of another culture. Lewis and Newbigin were missional apologists: each developed a missionary diagnosis of the cultural context and explored ways of...