REINHOLD NIEBUHR (1892-1971) began his career as a young pastor in Detroit. By 1928, when he moved to New York City to take a position at the Union Theological Seminary, he was a committed pacifist, and ran for Congress in 1932 as a Socialist. Breaking with his pacifist and socialist associates shortly after the outbreak of World War II in Europe, he articulated a realist political philosophy that aligned with his ever-evolving theology. Niebuhr’s thinking has influenced American culture in profound ways, whether through the simple but powerful “Serenity Prayer” known to millions or via the influence of his writings on social and political figures such as Martin Luther King, Jr. and Barack Obama. ELISABETH SIFTON, an editor and book publisher for forty years, is the author of The Serenity Prayer: Faith and Politics in Times of Peace and War. She lives in New York City and in Princeton, New Jersey.