Nothing divides the United States from Europe like religion. America has its public piety and its multitude of thriving sects, Europe has its official secularism and its empty, museum-piece churches. Ninety percent of Americans say they believe in God, while only about 60 percent of Britons, French, and Germans say the same. American politics is riven by faith-based disputes that barely exist across the Atlantic, while European debates take place under a canopy of unbelief that’s unimaginable in the United States, where polls show that a Muslim or a homosexual has a better chance of being elected president than an acknowledged atheist.

The aftermath of 9/11 has thrown this contrast into sharp relief and seemingly pushed Europe and America into permanent, Venus-and-Mars opposition. But paradoxically, our era may be remembered as the moment when the religious gulf between the continents began to slowly close. In the United States, the Bush era has summoned up—arguably for the first time in this country’s history—a mass secularism that looks to Europe and sees a model for America to follow. In Europe, meanwhile, a rising Islam and a more assertive Christian remnant are touching off American-style culture wars on a continent that had prided itself on being past those messy controversies.

America’s secular turn actually began in the 1990s, though it wasn’t until 2002 that two Berkeley sociologists first noticed it. In a paper in the American Sociological Review, Michael Hout and Claude S. Fischer announced the startling fact that the percentage of Americans who said they had “no religious preference” had doubled in less than 10 years, rising from 7 percent to 14 percent of the population. This unexpected spike wasn’t the result of growing atheism, Hout and Fischer argued; rather, more Americans were distancing themselves from organized religion as “a symbolic statement” against the religious right. If the association of religiosity with political conservatism continued to gain strength, the sociologists suggested, “then liberals’ alienation from organized religion [might] become, as it has in many other nations, institutionalized.”

Five years later, that institutionalization seems to be proceeding. It’s showing up in an increasingly secularized younger generation: A recent Pew Research Center survey found that 20 percent of 18-to-25-year-olds reported no religious affiliation, up from just 11 percent in the late 1980s. It’s visible on the best-seller lists, where books such as Kevin Phillips’s American Theocracy make their pitches to liberal readers, and in the public comments of scientists who now seem eager to attack religion as a threat rather than dismiss it as a nuisance. And it’s found a home in the expanding world of the liberal blogosphere, which has provided a virtual parish for Americans united by their disdain for “godbags” and “fundies.” (A Pew study of Howard Dean activists, one of the first mass constituencies mobilized by “netroots” activism, found that 38 percent described themselves as “secular.”)

It’s even making a difference at the ballot box. Liberals have spent much of the past six years straining to cut into the GOP’s advantage among religious voters. But when the Democrats finally shattered the Republican majority in the 2006 midterms, it was their consolidation of the secular vote that helped put them over the top. Despite all their efforts to close the God gap, the Democrats managed barely any gains among frequent churchgoers last November—but their share of the vote among Americans who never attend church at all leaped to 67 percent, from 55 percent in 2002.

This isn’t the first secular moment in American history; indeed, the modern religious right emerged, in part, as a reaction to what was perceived as the growing Godlessness of America’s political institutions, from the Supreme Court’s decision banning school prayer to the disproportionate influence of secular voters in the Democratic Party of the 1970s. But that earlier secularism tended to be an elite phenomenon: Even Time magazine’s famous 1966 cover story, “Is God Dead?,” confined its analysis to the world of intellectuals, and noted that in America, “public faith in God seems to be as secure as it was in medieval France.” The secularism that has come of age in the Bush era, by contrast, seems to have a greater mass appeal. What’s more, where the earlier secularism tended to cultivate a self-conscious neutrality toward religion, the new secularism is defined by an unabashed hostility toward traditional faith—or at least toward any attempt to mix such faith with politics. The more-extreme secular voices regard religion as a virus or a poison; more-nuanced polemicists merely argue that their religious opponents are un-American, and that faith-based politics is a stalking horse for theocracy.

This hard secularism is a relatively radical idea in the American context, where the separation of church and state has rarely separated God from politics: From abolitionist and “Social Gospel” activism in the 19th century to the civil-rights and anti-abortion movements of the 20th, religiously motivated political action has been a regular feature of our national life. Indeed, the America that many secularists seem to desire looks an awful lot like the Europe of today, where politicians who mention God are a rarity, and governments keep a wary eye on “sects” that stray too far outside the mainstream.

Yet the Europe of tomorrow may look more like … the United States, with a politics that’s increasingly shaped by clashes between believers, or between belief and unbelief. Already, the Continent is experiencing a low-grade culture war, created by the collision between the religious zeal of Muslim immigrants and the secular culture that surrounds them. In flash points that range from the murder of the anti-Islamic filmmaker Theo Van Gogh in Holland, to the controversy over the supposedly blasphemous Danish cartoons, to the question of whether to admit Turkey to the EU, secular Europe has found itself in unfamiliar, God-haunted, almost American territory. Such disputes may subside as Islamic immigrants assimilate to European norms, but for now, at least, resistance to assimilation by Muslims suggests that they may succeed in changing those norms as much as they are changed by them.

Meanwhile, there are signs that Christianity, too, is emerging from its decades-long defensive crouch. Pope Benedict XVI has taken the re-Christianization of Europe as a theme of his papacy, and his church’s recent interventions in Spain’s debate over same-sex unions and in Italy’s referendum on whether to loosen restrictions on in vitro fertilization bear an unmistakable resemblance to the gauntlet-throwing that Americans have come to expect from their churchmen.

In each case, demography may be pushing religion back into European politics. The Muslim birthrate in Europe is far higher than the birthrate among non-Muslims, and immigration from the Islamic world continues apace. Meanwhile, immigrants from Africa and Latin America have injected a new vitality into European Christianity, creating thriving Evangelical and Pentecostal communities in urban areas where many of the established churches stand empty. It was Christians’ demographic advantage in the ancient world, the sociologist Rodney Stark has suggested, that helped their faith take over Europe in the first place, and high fertility rates help explain the growth of evangelical Christianity and Mormonism in the United States over the last century. Now similar demographic forces, the political scientist Eric Kaufmann argued last year in the British magazine Prospect, may be “carrying Europe towards a more American model of modernity,” in which the wall of separation between church and state looks more like a picket fence, easily scaled or shimmied through.

These trends shouldn’t be exaggerated. America remains a deeply religious nation and its secularists an embattled minority, while Europeans remain strongly invested in preventing faith from intruding into politics. But both continents may be drifting into a zone where religious belief is likely to be a persistent source of tension, rather than a commonplace or a curiosity.

Religion stirs up the most controversy, a group of Harvard economists recently argued, when roughly half the population is actively religious; conflict ebbs when the devout constitute large majorities or small minorities. The more evenly divided a culture finds itself on the ultimate questions, the more likely politicians are to pursue “strategic extremism” and mobilize one side against the other. Precisely this kind of polarization dominated European politics from the French Revolution until the middle of the 20th century, sparking regular clashes— Germany’s Kulturkampf, France’s Dreyfuss Affair, Spain’s Civil War—between secular and religious ideologies.

America has long avoided this trap by enjoying near-universal piety; Europe, at least lately, has escaped it by cultivating near-universal skepticism. But if the religious gulf between the two continents narrows, the divides within each one are likely to open ever wider, and religious peace turn increasingly to culture war—or worse.