Niebuhr's conclusions about the ambiguity of human existence derived mostly from his beliefs regarding the nature of human beings and their relationship to higher realities, especially God. These beliefs, clearly evident in Moral Man and Immoral Society and The Children of Light and the Children of Darkness, reflected the technical civilization in which Niebuhr lived, yet they depended far more on what Niebuhr conceived to be timeless truths than on the current state of scientific knowledge.

Niebuhr's later writings placed greater emphasis on science and technology, for the good reason that in 1945 the world changed forever. The invention of atomic weapons threatened humanity with devastation on a scale unimagined before. The United States briefly enjoyed a monopoly of the new instruments of destruction, but no one who understood the physics involved expected the monopoly to last more than a few years. It didn't. The Soviet Union joined the atomic club in 1949, prompting the American government to seek a new circle of exclusivity. The Soviets followed close behind, and by the end of 1953 both Americans and Russians possessed hydrogen weapons. The development of long-range missiles during the next several years enhanced the danger still more, and conjured up the prospect that each side would soon be able to obliterate the other within minutes.

The nuclear menace added bite to Niebuhr's remarks about the ambiguity and inconclusiveness of human activities, and implicitly challenged the vindicationist consensus on which American policy during the early Cold War rested.