The tendency to claim God as an ally for our partisan value and ends is the source of all religious fanaticism.

Niebuhr is warning against the perennial temptation to enlist God as a guarantor of one’s own political or moral program. When a party, nation, or movement treats its aims as identical with God’s will, it becomes insulated from criticism and correction; opponents are no longer merely mistaken but impious. For Niebuhr, this fusion of divine authority with “partisan” ends produces fanaticism because it sacralizes contingent interests and converts faith into an instrument of power. The quote reflects his broader Christian realist insistence on human fallibility, the ambiguity of moral motives, and the need for humility in public life—especially when religious language is used to justify coercion or violence.