If asked to name the most influential work of modern Christian thought, older Protestant divines might point to Karl Barth's powerful commentary on The Epistle to the Romans or Paul Tillich's Systematic Theology. Younger ministers, on the other hand, would be far more likely to cite a book that is scarcely more than an elliptical fragment of theology, since it was never intended for publication at all. It is the Letters and Papers from Prison by Dietrich Bonhoeffer, the now-famed German Lutheran pastor who was arrested and later executed by the Nazis.
Some of Bonhoeffer's appeal for the young stems from...