According to Proust, not all people living at the same time are genuine contemporaries. Sensibility, culture, and education vary so widely that different people really belong to different eras, no matter what the calendar says. Thus a businessman living in 2006 might really be a 19th-century robber baron, while his son might be an aesthete whose true home is ancient Greece, or an ascetic who comes from the 12th century.

Sam Harris, whose “Letter to a Christian Nation” (Alfred A. Knopf, 96 pages, $16.95) is the latest entry in what has become the thriving mini-genre of anti-religious polemics, would surely endorse Proust’s principle, at least as far as his opponents are concerned. In this crude and disorganized pamphlet, Mr. Harris — whose 2004 book “The End of Faith” covered much the same ground, at greater length — generally seems to believe that anyone who calls himself a Christian belongs in the time of Moses and Joshua, intellectually and morally speaking. “Those with the power to elect our presidents and congressmen,” he writes, “believe that dinosaurs lived two by two upon Noah’s ark … and that the first members of our species were fashioned out of dirt and divine breath, in a garden with a talking snake, by the hand of an invisible God.” His gift-book-sized broadside is addressed to such a hypothetical Christian, and lays out, with mounting anger and contempt, the reasons why Mr. Harris considers him a dangerous fool.

It is no accident that Mr. Harris’s militant atheism conceives of Christianity in terms that any fundamentalist could endorse: It is a classic case of the meeting of extremes. For Mr. Harris, as for Pat Robertson, you are not really a Christian unless you believe in Biblical inerrancy, prefer Genesis to Darwin, and want America to become a theocratic state. Otherwise you are a mushy, muddle-headed “religious liberal,” and the one thing on which Mr. Harris and his imaginary antagonist can agree is that such trimmers deserve the contempt expressed for them in the Book of Revelation: “So then because thou art lukewarm, and neither cold nor hot, I will spew thee out of my mouth.” Only once the field is cleared of such bystanders can Mr. Harris come to grips with the essential questions: “Either Christ was divine, or he was not. If the Bible is an ordinary book, and Christ an ordinary man, the basic doctrine of Christianity is false.”

Mr. Harris regards himself, of course, as a 21st-century rationalist, and he is enraged by the atavistic ignorance of Christians who, for instance, oppose stem cell research:

The naive idea of souls in a Petri dish is intellectually indefensible. It is also morally indefensible, given that it now stands in the way of some of the most promising research in the history of medicine. Your beliefs about the human soul are, at this very moment, prolonging the scarcely endurable misery of tens of millions of human beings.

Rhetoric like this makes it clear, however, that the mental era to which Mr. Harris belongs is not the 21st century, but the 17th. Everything about “Letter to a Christian Nation,” with the exception of its atheism, would seem quite at home in a bookstall in Milton’s London. Like a Puritan pamphleteer, Mr. Harris is contemptuous, self-righteous, and insultingly personal. He thrills to each use of the second person: “Nonbelievers like myself … stand dumbstruck by you … by your denial of tangible reality, by the suffering you create in service to your religious myths, and by your attachment to an imaginary God.”

Clearly this kind of writing is not designed to persuade any actual Christian to abandon his beliefs. On the contrary, it takes only a slight understanding of human nature to recognize that insulting someone’s beliefs is the best way to reinforce them: amour propre will always stop us from surrendering our self-respect along with our principles. Mr. Harris’s real audience, the choir to whom he is preaching, is his fellow secularists, who feel threatened by what they perceive as an increasingly assertive and politicized Christianity. The popularity of anti-religious tracts like Susan Jacoby’s “Freethinkers” and Daniel Dennett’s “Breaking the Spell” is proof of secularists’ genuine feeling that they are under siege.

The problem, of course, is that the politicized Christians they oppose are motivated by just the same kind of defensiveness and resentment. The equally numerous recent polemics in defense of religion — such as Alister McGrath’s “The Twilight of Atheism,” or Michael and Jana Novak’s “Washington’s God” — are not written in a tone of crusading triumph, as though by a Church Victorious. On the contrary, they are fueled by the Christian Right’s feeling of alienation from American culture, both mass and elite, and by their sense that the U.S. government has become positively hostile to religion.

Atheists and believers cannot, of course, both be under siege. The truth is that America is probably much the same as it has ever been: both worldly and pious, secular and churchgoing, thanks to a benign cognitive dissonance. The polls cited by Mr. Harris, which show that “forty-four percent of the American population is convinced that Jesus will return to judge the living and the dead sometime in the next fifty years,” tell us less about the American character — which remains deeply materialistic, optimistic, and self-centered — than about the idleness of asking people to define their religious beliefs in a telephone poll. The uses people make of religion are very various, and have less to do with subscription to a set of dogmas than with intuitions and aspirations. Mr. Harris is right that, for each of us, intellectual honesty demands a full reckoning with the claims of religion; and he is right that a life without religion is not a life without meaning and moral purpose. But such a reckoning is difficult and often painful, and Mr. Harris’s deliberately obnoxious book can make no contribution to it.

The importance of Brooke Allen’s “Moral Minority” (Ivan R. Dee, 256 pages, $24.95) a much more substantial polemic, lies in its demonstration of the way the Founding Fathers conducted themselves as rationalists in an overwhelmingly Christian country. Ms Allen, best known as a literary critic, sets out here to refute the feeble attempts of some Christian conservatives to claim the Founders as their own. Quoting copiously from the writings of her six subjects — Franklin, Adams, Washington, Jefferson, Madison, and Hamilton — Ms. Allen shows that such claims are completely untenable. “The key founding fathers,” she writes, “were not strictly orthodox believers and in many cases not even Christians at all.” All of them were shaped by an Enlightenment Deism that saw religion as at best a source of moral teaching and social cohesion, at worst “an irrational, divisive, and atavistic passion.” The attitudes of these men towards Christianity received different inflections from their different personalities, but Franklin’s shrewd and politic agnosticism, Jefferson’s Voltairean scoffing, and Washington’s imperturbable indifference were equally far from Christian belief.

In her avowed purpose, then, Ms. Allen succeeds perfectly — as well she should, since she is a fine writer and the facts are all on her side. Yet “Moral Minority” does not dwell long enough on the implications of its title. The rationalism and skepticism of the Founders were distinctly aristocratic traits, accommodated by their social and economic position at the top of colonial society. Like aristocrats of all eras, but especially the 18th century, they had little need for religion and little use for it. Yet they recognized that the country they were forging was filled with believers of all kinds, some of them dismayingly fanatical. “What a mercy it is that these People cannot whip and crop, and pillory and roast, as yet in the U.S.! If they could they would,” John Adams wrote to Jefferson in 1817.

This recognition of the power of religion, even in a scientific age, is what made the Founders separate church and state so rigorously. And they found their strongest support, it is worth remembering, among minority Christian sects like the Baptists, who knew that a secular government was their best protection against the Congregationalist and Episcopal establishments. The political wisdom of the Founders was even greater than their religious wisdom, for they created an America where skeptic and believer could live together in peace. Despite the alarms raised these days on both right and left, they still do.