The mastery of nature is vainly believed to be an adequate substitute for self mastery.

Niebuhr contrasts modernity’s growing technical power—our ability to control and exploit the natural world—with the harder moral task of governing the self. The line reflects his characteristic “Christian realist” suspicion that progress in science, industry, or politics does not automatically produce virtue. Indeed, increased power can magnify pride, greed, and collective self-deception if it is not matched by humility, restraint, and ethical discipline. The quote thus warns against confusing external control (over nature) with inner freedom (over one’s impulses and ambitions), and it implies that societies that celebrate mastery and efficiency while neglecting character will misuse their achievements.