As I read celebrity atheist Christopher Hitchens’s recent Newsweek attack on the pope in particular and Roman Catholicism in general, I remembered an incident that happened when I was in the U.K. in early January. Walking out of London’s Victoria Station, I was stopped by a TV reporter who asked me what I thought about the British atheists’ newest ad campaign. It was one of those typical man-in-the-street interviews, with a reporter and a cameraman buttonholing passersby to find a snappy quote for the evening news.

In England, which has long been a cultural template for the U.S., the atheists, after years of calling themselves humanists, have finally come out of the closet. With strong support from the renowned Oxford zoologist Richard Dawkins, the new campaign has splashed an ad on the side of 800 British buses proclaiming, “There is probably no God. Now stop worrying and enjoy your life.” Immediately following the ads came an announcement from the BBC early last month that it would add atheists to the list of various people of faith who are invited to offer the three-minute “Thought for the Day” on the influential Radio 4.

Britain has actually recognized atheism for some time now. As a country with an officially established church, it requires all its state primary schools to include religious-education classes. The classes often reflect the ethnic and religious composition of the schools’ surrounding neighborhoods, so that those in heavily Muslim or Hindu communities will focus largely on non-Western religious traditions. Yet one mandate of all these classes involves introducing students to religious diversity and pluralism rather than teaching any specific dogma. In 2004, the government decided that pluralism requires that all schools include some instruction on atheism.

While figures for the U.S. suggest that 5% to 10% of our population claims not to believe in God, the comparable numbers in England are 35% to 40%. We should keep in mind, however, that, like surveys about sexual practices, body art or TV habits, such figures are always suspicious. When it comes to our private lives, how many of us conceal or exaggerate?

Back in the States, the closet door on adamant disbelief may not yet be open, but President Barack Obama’s inaugural address certainly began turning the doorknob. For the first time in history, a president publicly acknowledged atheists by including “nonbelievers” in our “patchwork heritage” of “Christians and Muslims, Jews and Hindus.” To make sure no one missed his point, he spoke admiringly of “humanists” and “those who subscribe to no faith” in his comments a few weeks later at, of all places, the National Prayer Breakfast.

Our new president’s cautious phrasing may suggest that the country is not yet ready for the full debut of atheists, but they are certainly claiming a louder voice in the culture wars. And that voice sounds very different from the bizarre theatrics of Madalyn Murray O’Hare. A group once relegated to the edges of our culture, to college student unions and late-night cable, is now poised for prime time.

The somewhat aging enfant terrible Christopher Hitchens, author of an oddly dyspeptic attack on Mother Teresa (“The Missionary Position”) and the recent bestseller “God Is Not Great: How Religion Poisons Everything,” is simply the most public face of American atheism. Also on the bestseller list in the past have been Sam Harris’s “Letter to a Christian Nation” and Richard Dawkins’s “The God Delusion.” And now, behind the scenes, groups like American Atheists, the Freedom From Religion Foundation and the Council for Secular Humanism have been busy publishing journals, funding college scholarships and establishing Web sites.

They are also venturing cautiously into advertising, with public campaigns in cities as diverse as Denver, Washington, Philadelphia and Charleston, S.C. Just in time for this year’s Lenten season, an ad in gaudy Mardi Gras colors on the side of a New Orleans streetcar proclaims, “Don’t Believe in God? You Are Not Alone.”

So far, American atheists have no figurehead with the brilliance or literary and scientific prizes of Britain’s Mr. Dawkins, the recently retired Simonyi Professor of the Public Understanding of Science at Oxford, where Balliol College named one of its most prestigious awards after him. Even so, these new American atheists are far better advocates for their cause than the dysfunctional O’Hare clan. Now that they have broken the ice, in fact, we should only hope that even more thoughtful atheists will follow them into the pool.

Why should believers welcome this emergence of unbelief? Why not? We should be glad that there are people, even the devil’s disciples, who take religion seriously enough to attack it, especially in these days when God seems to appear only in quarrels over holiday displays, during political campaigns or on the self-help shelves of Barnes & Noble. Should the primary goal of religion really be to fund municipal crèches, allow politicians to end every speech with the tag “And God bless America,” or inspire works like “Tea With God: A Divinely Inspired Self-Help Book” and “The Christian Entrepreneur: How to Profit From Your God-Given Idea”?

In attacking the cloistered monks and nuns of my Roman Catholic Church, the brilliant, if occasionally logorrheic, John Milton wrote in his defense of free speech, “Areopagitica,” that “I cannot praise a fugitive and cloistered virtue, unexercised and unbreathed.” And what will possibly make us exercise and breathe more fully than challenges by intelligent, thoughtful opponents?

Even my Jesuit teachers admitted, at times grudgingly, that the Protestant Reformation re-energized the Catholic Church by forcing it to respond to Luther’s call for religion to engage the world rather than separate from it. While some who trumpet their faith may be a little too eager to engage in petty squabbles, we should be able to expect our leaders in all fields to articulate how their beliefs shape their vision, values, actions and policies.

And if we truly believe that an open, vigorous marketplace of ideas will establish value and truth as clearly as honest and open economic markets, shouldn’t we encourage everyone to enter that market?

I told the London reporter at Victoria that I admire people who take religion seriously enough to challenge it. And I suspect God would too, if he thought ads on the sides of buses or atheist thoughts for the day were as worthy of his time as helping people find meaning in their lives and peace in their souls. Perhaps if we are confronted with better questions about the meaning and value of religion, we will be forced to find better answers.

Mr. O’Sullivan, author of “The Books of Job,” is Kenneth Curry Professor of Literature at Rollins College.