If we can find God only as he is revealed in nature we have no moral God.

Niebuhr is warning against a purely “natural theology” that tries to infer God’s character directly from the observable world. Nature displays grandeur and order, but also indifference, waste, and violence; if that is our only revelation, the “God” we derive will mirror amoral natural processes rather than provide a standard for justice, mercy, or obligation. For Niebuhr, a morally authoritative God must be known through a disclosure that judges and redeems human life—classically, the biblical and especially Christian revelation—rather than through nature alone. The line fits his broader critique of sentimental optimism and his insistence that moral meaning cannot be read straightforwardly off history or biology.