The recent outburst of popular religiosity in the United States is a most dramatic and unforeseen development in American life. As Europe grows more secular, America grows more devout.

In the midst of this religious commotion, the name of the most influential American theologian of the 20th century rarely appears - Reinhold Niebuhr. It may be that most "people of faith" belong to the religious right, and Niebuhr was on secular issues a determined liberal. But left evangelicals as well as their conservative brethren hardly ever invoke his name. Jim Wallis's best-selling "God's Politics," for example, is a liberal tract, but the author mentions Niebuhr only twice, and only in passing.

Niebuhr was born in Missouri in 1892, the son of a German-born minister of the German Evangelical Synod of North America. He was trained for the ministry at the Synod's Eden Theological Seminary and at the Yale Divinity School. In the 1920's he took a church in industrial Detroit, the scene of bitter labor-capital conflict. Niebuhr's sympathies lay with the unions, and he joined Norman Thomas's Socialist Party. Meanwhile, New York's Union Theological Seminary, impressed by the power of his preaching and his writing, recruited him in 1928 for its faculty. There he remained for the rest of his life. He died in 1971.

Why, in an age of religiosity, has Niebuhr, the supreme American theologian of the 20th century, dropped out of 21st-century religious discourse? Maybe issues have taken more urgent forms since Niebuhr's death - terrorism, torture, abortion, same-sex marriage, Genesis versus Darwin, embryonic stem-cell research. But maybe Niebuhr has fallen out of fashion because 9/11 has revived the myth of our national innocence. Lamentations about "the end of innocence" became favorite clichés at the time.

Niebuhr was a critic of national innocence, which he regarded as a delusion. After all, whites coming to these shores were reared in the Calvinist doctrine of sinful humanity, and they killed red men, enslaved black men and later on imported yellow men for peon labor - not much of a background for national innocence. "Nations, as individuals, who are completely innocent in their own esteem," Niebuhr wrote, "are insufferable in their human contacts." The self-righteous delusion of innocence encouraged a kind of Manichaeism dividing the world between good (us) and evil (our critics).