There is no cure for the pride of a virtuous nation but pure religion.

Niebuhr is warning that nations (especially those that see themselves as morally exemplary) are prone to a distinctive form of collective pride: the conviction that their power is inherently righteous. In his thought, this “virtue” can become a mask for self-interest, making a people less capable of self-critique and more willing to justify coercion as moral duty. By “pure religion,” he means a faith purified of national self-congratulation—one that emphasizes humility, repentance, and the limits of human righteousness. The line thus presses a political-ethical point: only a transcendent moral standard that judges the nation as well as its enemies can restrain the moral arrogance that accompanies perceived virtue.