There is one thing that is not allowed in American national politics – and that is atheism. “In God We Trust” is on the currency; and the number of congressional members who avow no faith at all are about as plentiful as those who are openly gay (none in the Senate; five in the House).

Under the last president, religious faith – evangelical Christianity or Benedict-style Catholicism – was a prerequisite for real access to the inner circle. But the requirement is not just Republican. Among the more excruciating campaign events of last year was a faith summit for the Democrats in which candidates vied with one another to express the most piety. Barack Obama’s Christianity – educated, nuanced, social – is in many ways more striking than that of, say, Nixon, Truman or Eisenhower.

Americans are losing faith, though; and those who have it are moving out of established churches. The nonreligious are now the third biggest grouping in the US, after Catholics and Baptists, according to the just-released American Religious Identification Survey. The bulk of this shift occurred in the 1990s, when they jumped from 8% to 14% of the population – but they have consolidated in the past decade to 15%.

As elsewhere in the West, mainline Protestantism has had the biggest drop – from 19% to 13%. Despite heavy Latino immigration, the proportion of Catholics has drifted down since 1990, and their numbers have shifted dramatically from the northeast and the rust belt to the south and west. Take South Carolina, a state you might associate with hardcore Protestant evangelicalism. It certainly does exist there – but in that southern state, the percentage of Catholics has almost doubled since 1990 and the percentage of atheists has tripled.

America, it turns out, is a more complicated spiritual place than the stereotypes might imply. Islam is still tiny – and integrated and largely successful. Catholicism, while buoyant among new Hispanic immigrants (who are, nonetheless, drifting rapidly towards evangelicalism in the southern hemisphere whence they came), has plummeted in its heartland. Think of Massachusetts, the home of the Irish and Italian and Portuguese. In 1990, Catholics accounted for 54% of all residents of the Kennedys’ state. That’s now 39%.

The bulk of these ex-Catholics joined no other faith group – and the number of residents claiming no religion at all jumped from 8% to 22%. Of course, the sex abuse scandal played a powerful part. One of the chief enablers and protectors of abusive priests, Cardinal Bernard Law, was based in Boston and escaped real accountability by being given a prestigious sinecure in Rome. The Irish and Italians in Massachusetts did not forget.

In many ways the most interesting dynamic is that between mega-church, politicised evangelicalism and atheism. Mega-churches have emerged in many suburban neighbourhoods in America and serve as community centres, as social-work hubs and as venues for what most outsiders would think of as stadium-style Sunday rock shows, in which religion looks like a form of fandom. Charismatic preachers – like the now disgraced Ted Haggard or the politically powerful Rick Warren – have built massive congregations.

The movement has spawned its own shadow pop music industry, coopts the popular culture as any brand-conscious franchise would and has a completely informal form of worship. Go to one of these places and it feels like a town in itself – with shops, daycare centres, conference rooms and social networking groups. The car parks feel like those in sports stadiums; and the atmosphere evokes a big match. In 20 years, the number of Americans finding identity and God in these places has soared from 200,000 to more than 8m.

This is not, one hastens to add, an intellectual form of faith. It is a highly emotional and spontaneous variety of American Protestantism and theologically a blend of self-help, biblical literalism and Republican politics. This is, in many ways, how George W Bush reframed conservatism in America – and with one in three Americans now calling themselves evangelical, you can see the political temptation. The problem was that the issues the evangelicals focused obsessively on – abortion, gays, stem cells, feeding tubes for those in permanent vegetative states – often came to seem warped to many others. Those who might once have passively called themselves Christian suddenly found the label toxic, if it meant identifying with such a specific political agenda. And so as evangelicalism rose, atheism and nonaffiliation emerged as a reaction.

It is impossible to know where this is heading, but the latest survey is a reminder to exercise a little scepticism when you hear of America’s religious exceptionalism. Yes, America is far more devout than most of western Europe; but it is not immune to the broader crises facing established religion in the West. The days when America’s leading intellectuals contained a strong cadre of serious Christians are over. There is no Thomas Merton in our day; no Reinhold Niebuhr, Walker Percy or Flannery O’Connor. In the arguments spawned by the new atheist wave, the Christian respondents have been underwhelming. As one evangelical noted in The Christian Science Monitor last week, “being against gay marriage and being rhetorically pro-life will not make up for the fact that massive majorities of evangelicals can’t articulate the Gospel with any coherence”.

The quality of the Catholic priesthood has also drifted downward: the next generation of priests is more orthodox, but also more insular and less engaged with the wider world. There are a few exceptions: the 29-year-old orthodox Catholic Ross Douthat has just won a treasured opinion column slot in The New York Times. But he is sadly an exception that proves a more general rule. American Christianity may be stronger in some pockets, but it is dumber too. In the end, in the free market-place of ideas and beliefs, that will count.

What one yearns for is a resuscitation of a via media in American religious life – the role that the established Protestant churches once played. Or at least an understanding that religion must absorb and explain the new facts of modernity: the deepening of the Darwinian consensus in the sciences, the irrefutable scriptural scholarship that makes biblical literalism intellectually contemptible, the shifting shape of family life, the new reality of openly gay people, the fact of gender equality in the secular world. It seems to me that American Christianity, despite so many resources, has ignored its intellectual responsibility. And atheists, if this continues much longer, will continue to pick up that slack.

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