The sad duty of politics is to establish justice in a sinful world.

The line distills Niebuhr’s “Christian realism”: politics is necessary because human beings and societies are morally compromised, yet it remains morally urgent because injustice is real and remediable. Calling the task “sad” signals that political action is never pure—power, coercion, and compromise are unavoidable in collective life. Still, the aim is not utopian perfection but proximate justice: restraining evil, protecting the vulnerable, and building workable institutions in a world where sin (self-interest, pride, group egoism) distorts intentions and outcomes. The quote thus rejects both cynical power politics (justice is impossible) and naïve idealism (justice is easy), insisting on responsible engagement under tragic conditions.