We live in an age of certainty. The progressive left is certain of its righteousness. The nationalist right is certain of its innocence. The technologist is certain that the right platform will finally solve the ancient problem of human nature. And our democracies — fragile, fractured, besieged by forces that their founding mythologies never adequately prepared them to resist — are paying the price of certainties that history has already indicted and that we refuse, with a stubbornness that would be impressive if it were not so costly, to reexamine. Reinhold Niebuhr saw all of this coming. Writing from a small Detroit parish in the 1920s, teaching at Union Theological Seminary through the catastrophic middle decades of the twentieth century, shaping the thinking of George Kennan, Arthur Schlesinger, John F. Kennedy, and Barack Obama, America's greatest public theologian spent fifty years building a framework for understanding the human situation that is, in the specific conditions of the twenty-first century, more urgently relevant than anything written since his death in 1971. He called it Christian Realism. We might simply call it the truth about ourselves — the truth that our political culture has been most systematically and most consequentially avoiding.