Reinhold Niebuhr was a twentieth-century American theologian who was known for his commentary on public affairs. One of his most influential ideas was the relating of his Christian faith to realism rather than idealism in foreign affairs. His perspective influenced many liberals and is enjoying a resurgence today; most recently Barack Obama has acknowledged Niebuhr's importance to his own thinking.

In this book, Kenneth Hamilton makes a claim that no other work on Niebuhr has made--that Niebuhr's chief and abiding preoccupation throughout his long career was the nature of...