REINHOLD NIEBUHR: A Biography. By Richard Wightman Fox. Pantheon. 340 pp. $19.95.

BY NECESSITY any biography of Reinhold Niebuhr is also a piece of American intellectual history. For at least two generations, until his death in 1971, the great theologian was virtually the liberal conscience incarnate. He was religious liberalism's main American public figure, the "crisis theologian" of many a cause, from opposition to the Cold War to Vietnam protests -- a veritable commander of the liberal flagship.

Never has his life been so richly delineated as Richard Fox has drawn it here. Fox has written not only an excellent biography but a notable chapter in the history of American religious thought. It is also a notable feat of the imagination. Fox never saw or heard Niebuhr. Yet his portrayal of the man will surely strike many readers as executed from life. The moral passion, the formidable intellectual energy, the drive for domination, even the idiosyncratic pulpit and rostrum gestures, are all there. This was a man who created a powerful illusion of being larger than life.

Fox also does a superb job of evoking the midwestern boyhood home where Niebuhr's perception of himself was generated. And he makes a good case for the persistent potency of his father's influence. During the early years of World War I, Niebuhr was a confessedly humble pastor of a comparatively obscure church in Detroit. He was providentially discovered by leading lights in the liberal firmament, such as Charles Clayton Morrison, founder of The Christian Century, but above all by Sherwood Eddy, then the most prominent spokesman for the liberal conscience.

It was Eddy who engineered Niebuhr's escape from parochial obscurity into national prominence. Eddy recognized in Niebuhr an exceedingly rare talent. He provided the funds for Niebuhr's appointment to the faculty of Union Theological Seminary in New York City, then arguably the finest theological school in this country. None of the prophets in ancient Israel was blessed with so generous a sponsor, and none of them was mentioned by heads of a great nation as someone they had read or at least read about. In the 1960 primary campaign Kennedy had a special consultant to talk to him about Niebuhr. Although Billy Graham has often been represented as spiritual adviser to presidents, and Jerry Falwell has shagged foul balls for Reagan, neither Graham nor Falwell counts as a theologian. And Niebuhr was a theologian.

There are significant ambiguities in Niebuhr's work as a theologian. Fox carefully pursues these, but not always with the success of his biographical venture. He sees the youthful Niebuhr as having been deeply influenced by Douglas Clyde Macintosh, the Yale theologian-philosopher under whose direction Niebuhr wrote a master's thesis. Fox is correct in reporting that Macintosh's philosophical sophistication was wasted on Niebuhr, especially Macintosh's deep interest in epistemology. Apparently what Niebuhr did acquire under that tutelage was the pragmatic element in Macintosh's theology.

A considerable part of Niebuhr's later career was devoted to tilting with philosophers -- ancient, medieval, modern and contemporary. He charged headlong at rationalists, naturalists, idealists, mostly because of their misreadings of the human condition. He rejected dualism, and with very modest success he attacked substance metaphysics, perhaps under the influence of his reading of Alfred North Whitehead. He assailed supernaturalists as well as naturalists. But where did he come out? What was the positive doctrine?

Eventually he called it the Biblical view of man. Therein he stressed the unitary nature of human beings. Man is not soul plus body. He is spirit-and-body. Instinct invades spirit. Spirit has, surely, not only a monitorial but also a creative function in/over life.

Fox has Niebuhr's early theology markedly interested in "persons." So, presumably, is any moralist. But many liberals were philosophical Personalists, i.e., they believed that reality is made up of selves and their relationships. The material world is, accordingly, a kind of systematic illusion. This does not mean that nature need not be taken seriously. It does mean that spirit is the transcendent reality. Niebuhr never abandoned this principle.

At other points he forced momentous rifts in the liberal ranks. For instance, he adopted, for a while, Marxian-like views -- not that he was ever enamoured of Stalin's subversion of Marx's teachings. Niebuhr also claimed that liberals did not reckon properly with class conflict and the struggle for power in America. Nor in the '30s did they reckon properly with the demonic forces driving western civilization towards another war, which in 1933 Niebuhr believed was due in the early future and was bound to be "suicidal."

BUT NONE of this meant then or later that he had deserted the liberal cause. The Christian calling is to work for justice under the impetus of altruistic love. Not that a truly and persistently just society, or even a worldwide ethical commonwealth is a realizable goal in history. Thus he broke decisively with the perfectionist element of liberalism.

I suppose that today it is Niebuhr's belief in original sin that most people think of when they take his measure as a theologian. It marks a return, of a sort, to Augustinian theology. Whatever the influence of Kierkegaard and of Tillich, it was Niebuhr who for the American constituency made the Fall indispensable for the interpretation of history. Niebuhr sought to restore the symbol and concept of original sin to a primary place in Christian reflection. His treatment fairly bristles with Pauline resonances. Thus radical evil is not merely a penchant for breaking rules. Neither is it a fatal thirst for low-grade gratifications. The first sin is egocentricity, first not in time but in historical universality. Thence Niebuhr proceeded to develop his powerful analysis of self-righteousness and anxiety as constituent elements of human existence.

But what of grace? It has been widely believed that Niebuhr's theology does not get beyond Good Friday. That judgment does not sufficiently reckon with his conviction that there is a divinely ordained and divinely empowered existence beyond history, symbolized by the Resurrection. That is the transcendental hope. It is supported by faith as surely in Niebuhr's account of it as in Augustine's or Luther's. So also is it supported by love.

Accordingly Fox's judgment that Niebuhr was really a "thorough-going naturalist" beneath a veneer of traditional Christian imagery simply cannot be sustained. But not simply because Fox confuses naturalism with empiricism. Niebuhr's "ultimate appeal" was not to "the observed facts of human existence," it was to God's grace. Like all theologians, Niebuhr appealed to experience, but that did not make him an empiricist. He professed to be a hard-core political realist, but he was not any kind of a materialist. His perception of the human condition and his vision of the ultimate community were incurably paradoxical. Niebuhr knew that. Indeed he gloried in it.