The 1940s were a remarkably active and productive decade for Reinhold Niebuhr. Two of his most important books were published during this period, The Nature and Destiny of Man and The Children of Light and Children of Darkness. He worked with others to establish a new journal, Christianity and Crisis. Alongside this he was instrumental in setting up two campaigning organizations, the Union for Democratic Action and the Americans for Democratic Action. This chapter provides a historical overview of the decade detailing these achievements. It then examines Niebuhr’s contribution to the nascent ecumenical movement, in particular his chapter on social ethics written for the Oxford Conference of 1937, his address to that conference, and then his preparatory chapter and address to the first Assembly of the World Council of Churches in Amsterdam 1948. Niebuhr argued that the underlying cause of the global crises was secularism, in response to which the Church should preach a message of redemption.