In a country where more than eight in 10 people regard themselves as religious, it takes more than a little guts to preach about a world without God. But that's the message that is creeping across America, spreading ripples of dissent in its wake.

From Tampa in Florida, to Cincinnati, Ohio, and all the way across to Sacramento in California, billboards have been cropping up with messages that run across the grain of America's normally devout discourse. "Don't believe in God? You're not alone!" were the first posters to be put up, in Arizona, Colorado, Texas and parts of the north-east. "Being a good person doesn't require God," read another.

The billboards are the work of a national group of atheists – or nontheists, as they call themselves – called United Coalition of Reason that seeks to encourage nonbelievers throughout America by bringing them together.

Through their website, they set themselves up in the mould of a religious community, outlining their "mission" , which they define as raising the visibility and sense of unity of the "community of reason".

The idea for the billboard campaign emerged after a Christian businessman in Tampa spent $50,000 (£32,000) of his own money putting up posters denouncing the separation of church and state . The coalition decided to take him on directly by spending twice as much on its own campaign.

The result has been predictably volatile. Its latest poster – "Are you good without God? Millions are" – has been defiled in Sacramento by religious opponents adding the words: "Also lost?"

In Cincinnati, the owner of the hoarding took down the poster after he received threats. And in Tampa, the believers have hit back with another campaign pouring scorn on the coalition with the help of quotations from George Washington.