About 400 people are preparing to gather for a conference in Hartford, Connecticut, to promote the end of religion in the US and their vision of a secular future for the country.

Those travelling to the meeting will pass two huge roadside billboards displaying quotes from two of the country's most famous non-believers: Katharine Hepburn and Mark Twain. "Faith is believing what you know ain't so," reads the one featuring Twain. "I'm an atheist and that's it," says the one quoting Hepburn.

At the meeting, members of the Freedom From Religion Foundation (FFRF) will hear speakers celebrate successes they have had in removing religion from US public life and see awards being presented to noted secularist activists.

The US is increasingly portrayed as a hotbed of religious fervour. Yet in the homeland of ostentatiously religious politicians such as Michele Bachmann and Rick Perry, agnostics and atheists are actually part of one of the fastest-growing demographics in the US: the godless. Far from being in thrall to its religious leaders, the US is in fact becoming a more secular country, some experts say. "It has never been better to be a free-thinker or an agnostic in America," says Annie Laurie Gaylor, co-president of the FFRF.

The exact number of faithless is unclear. One study by the Pew Research Centre puts them at about 12% of the population, but another by the Institute for the Study of Secularism in Society and Culture at Trinity College in Hartford puts that figure at around 20%.

Most experts agree that the number of secular Americans has probably doubled in the past three decades – growing especially fast among the young. It is thought to be the fastest-growing major "religious" demographic in the country.

Professor Barry Kosmin of Trinity College, who conducts the national Religious Identification Survey, believes up to a quarter of young people in the US now have no specific faith, and scoffs at the idea, prevalent in so much US media and culture, that the country is highly religious or becoming more so. "The trending in American history is towards secularisation," Kosmin said.

He cites the example of the changing face of Sunday in the country. It was not too long ago when many sporting events were banned on Sundays and most shops were closed too. Now the opposite is largely true.

As in Britain, Sunday in the US has become a normal shopping day for many, or a day to watch big football or baseball games. "The great secular holiday in America is Super Bowl Sunday. Even in the deep south, the biggest mega-church changes its schedule to suit the Super Bowl," Kosmin said.

He also pointed to social trends – greater divorce rates, gay marriage and much higher percentages of people having children out of wedlock – as other signs that the religious grip on society has loosened.

There are other indications, too. For a long time studies have shown that about 40% of US adults attend a church service weekly. However, other studies that actually counted those at church – rather than just asking people if they went – have shown the true number to be about half to two-thirds of that figure.

More Americans are now choosing to get married or be buried without any form of religious ceremony. At universities, departments devoted to the study of secularism are starting to appear. Books by atheist authors are bestsellers. National groups, such as the Secular Coalition of America (SCA), have opened branches across the country.

Herb Silverman, president of the Washington-based SCA, lives in Charleston, South Carolina. His local secularist group was founded in 1994 with 10 people, but now has 150 members. "I've been living here in the buckle of the Bible belt since 1976 and things are getting a lot better," Silverman said.

Yet there is little doubt that religious groups still wield enormous influence in US politics and public life, especially through the rightwing of the Republican party. Groups such as Focus on the Family are well-funded and skilful lobbyists.

Kosmin said the attention paid by politicians and the media to religious groups was not necessarily a sign of strength. "When religion was doing well, it did not need to go into politics. Secularity of our population and culture is obviously growing and so religion is on the defensive," he said.

However, it is still a brave US politician who openly declares a lack of faith. So far just one member of Congress, Californian Democrat Pete Stark, has admitted that he does not believe in God.

"Privately, we know that there are 27 other members of Congress that have no belief in God. But we don't 'out' people," said Silverman.

Others think that one day it will become politically mainstream to confess to a lack of faith as US political life lags behind the society that it represents. "Politicians have not yet caught up with the changing demographics of our society," said Gaylor.