American Protestant theologian, Union Theological Seminary professor, and the most publicly consequential American theological voice of the mid-20th century — read by presidents, quoted in political speeches, and recognized by secular readers as a serious political thinker in a way few theologians have managed before or since. Niebuhr began as a pastor in industrial Detroit, where contact with the realities of Ford's assembly line knocked the liberal Protestant optimism out of him, and he spent the rest of his career building a political theology adequate to the 20th century's catastrophes.
His signature position — "Christian realism" — rejected both the confident social gospel (which thought a reformed Christian society could abolish sin in human affairs) and political quietism (which left the world to the devil). Humans, Niebuhr argued, are genuinely capable of love and justice, but also inveterately prone to self-love, group loyalty, and the idolatry of their own projects. Politics is therefore the management of competing sinful interests through structures of power that check one another — an argument that gave him a near-tragic, balance-of-power politics with unmistakable affinities to Augustine and Madison. Moral Man and Immoral Society (1932) was the first crisp statement; The Nature and Destiny of Man (1941–43), his Gifford Lectures, the most systematic.
Niebuhr wrote the Serenity Prayer. He advised Cold War liberals of the ADA variety. King read him carefully (as did W. H. Auden), though King's Gandhian commitments ultimately outran Niebuhr's more guarded assessments of the possibilities of non-violence. Hans Morgenthau, George Kennan, and Arthur Schlesinger Jr. drew on him to shape mid-century American "realist" foreign policy.
His younger brother H. Richard Niebuhr was a theologian of comparable seriousness and rather different temperament.