It was a pretty avant-garde notion for the 18th century. And even, it seems, for the 21st, at least in certain regions of the world, some of them within our own borders. It hardly matters that my friend, a history professor, knows what he is talking about. Fundamentalist groups circulate leaflets containing stock responses to such arguments - including quotations that, torn from context, "prove" that the separation of church and state was never a basic American value. (After all, even the least orthodox of the Founding Fathers occasionally said something nice about Jesus.)

All of this is, in effect, just the latest chapter in the story told by Susan Jacoby in Freethinkers. It is a lively, if sometimes one-dimensional, survey of those Americans determined to keep God and government at a safe distance from one another. The expression lending her book its title has a slightly old-fashioned quality: Calling oneself a "freethinker" today is rather like capitalizing abstraction nouns, such as Reason or Progress. Her book rings with echoes from the lecture halls of 19th century America, where religious skepticism had its place alongside the causes of abolitionism, free love, women's suffrage and a shorter workday.

Jacoby's argument is that all such movements embodying Reason and Progress were influenced, if not guided, by secularists. The latter term subsumes agnostics, atheists, anti-clericalists and those whose religious beliefs coexist with an "insistence on the distinction between private faith and the conduct of public affairs." The role such figures have played in American history has been erased from public memory by what Jacoby often calls the "religiously correct" mentality -- a turn of phrase that would be slightly witty, if used no more than once, but that soon proves irksome.

So I grumble. But its tics notwithstanding, Freethinkers provides a readable chronicle of the ebb and flow of American commitment to the divorce between political and religious authority. Jacoby conveys the essential radicalism of the Constitution, which asserted that the new government was formed by and for a sovereign "We, the People," with no reference to God as the ultimate authority.

Religious leaders protested this at the time. Nor were they happy when Jefferson declared, "The legitimate powers of government extend only to such acts as are injurious to others. But it does me no injury for my neighbor to say there are 20 gods, or no God. It neither picks my pocket nor breaks my leg." And when Thomas Paine went from writing the pamphlets that fueled the revolutionary cause of 1776 to publishing works of deist philosophy, he was practically written out of the story of the founding of the republic.

And so secularism plays a dual role in Jacoby's telling of history. It is built into the infrastructure of society itself (at least insofar as the Constitution serves as the founding gesture by which the United States defines itself). And yet, it also is a counterculture, an underground. Her chapters on the 19th century are devoted to tracing the influence of secularism on the dissident movements of the day. Abolitionist editor William Lloyd Garrison, for example, read Paine's The Age of Reason; and in the 1890s, Elizabeth Cady Stanton, militant suffragist, published a skeptical work called The Woman's Bible that probably would earn her death threats if she went on talk radio today.

When Jacoby recounts the lives and struggles of freethought journalists and orators of the 19th century, her pages have a nostalgic quality. I suspect that their defiance and courage is only part of it. What her pages record is a crucial moment in the history of American mass culture -- a period when books and magazines were reaching people who, a generation or two earlier, might never have had the money or the literacy to read them. They could become curious about someone like Robert Ingersoll (the greatest orator of his day) and go to hear him denounce religion in a crowded lecture hall.

For people who could not take access to education and information for granted, learning was a source of pleasure. (Today, of course, media-saturated students arrive in the classroom expecting to be entertained. But that is a different matter altogether.)

And so most of the 20th century proves, in Jacoby's telling, a bit of a let-down -- one long backlash by fundamentalism against the godlessness of the Constitution, during which unbelievers had to make a tactical alliance with religious liberals to preserve the separation of church and state. As a result, she complains, the willingness of secularists to criticize religion itself (to denounce it as superstition and foolishness with anything like the forthrightness that Ingersoll showed) has diminished. Civil society is dominated by what Jacoby calls "the fog of murky tolerance that has since become the defining characteristic of America's ecumenical public consensus."

Someone with a less polemical feeling for American history is bound to have any number of reservations about Jacoby's way of reconstructing the past. To give just one example, it would have benefited from a careful reading of Ronald L. Numbers' Darwinism Comes to America (1998), which shows that the religious response to evolution was much more complex than her account suggests. And as engaging as the account of the 19th century cultural underground may be, Jacoby tends to confuse anti-clericalism and religious unorthodoxy with disbelief -- which makes it very uncomfortable for her when some of the rebels she admires begin attending seances.

Despite which, Freethinkers will make its contribution. For there are ten thousand letters to the editor, denouncing fundamentalism, waiting to be written -- with quotations from the Founding Fathers, supplied by Susan Jacoby.