[Niebuhr's] emphasis on sin startled my generation, brought up on optimistic convictions of human innocence and perfectibility. But nothing had prepared us for Hitler and Stalin, the Holocaust, concentration camps and gulags. Human nature was evidently as capable of depravity as of virtue ... Traditionally, the idea of the frailty of man led to the demand for obedience to ordained authority. But Niebuhr rejected that ancient conservative argument. Ordained authority, he showed, is all the more subject to the temptations of self-interest, self-deception and self-righteousness. Power must be balanced by power.

https://www.nytimes.com/1992/06/22/opinion/reinhold-niebuhr-s-long-shadow.html

https://en.m.wikipedia.org/wiki/Reinhold_Niebuhr

https://en.m.wikipedia.org/wiki/Christian_realism