"That's one of my favourite phrases in the book," says Daniel Dennett, his huge bearded frame snapping out of postprandial languor at the thought of it: "If you have to hoodwink your children to ensure that they confirm their faith when they are adults, your faith ought to go extinct." The 64-year-old Tufts University professor is amiable of aspect, but the reception he has had while in Britain promoting his new book, Breaking the Spell: religion as a natural phenomenon, has not been uniformly friendly. His development of the theory that religion has developed as an evolutionary "meme", a cultural replicator which may or may not have a benign effect on those who transmit it, has drawn attacks, not least in these pages, where John Gray accused him of "a relentless, simple-minded cleverness that precludes anything like profundity".

But Dennett has allies. In recent times he and other non- believers - what one might call a movement of "the New God-less" - have been girding their loins to do battle with the forces of increasingly intolerant and aggressive religions. If anyone doubts the need for combat, evidence is provided by the foothold gained in public discourse by "intelligent design", a dressed-up version of creationism hardly heard of a couple of years ago.

The New Godless is a broad movement, ranging from the militant atheism of Richard Dawkins, whose hardline Darwinism is familiar to millions from his presence on the airwaves and in print, to the softer approach of those who cleave to religion for cultural or aesthetic purposes while forsaking its supernatural component. They go by different names, and set out varying manifestos.

The British Humanist Association, whose long list of supporters includes Colin Blakemore, Michael Atiyah, Ian McEwan, Salman Rushdie, Claire Rayner, Jane Asher, Baroness Blackstone and Lord Dubs, declares its core principle to be that we can "live good lives without religious or superstitious beliefs". Basing their decisions on "the available evidence", the humanists recognise that "moral values are properly founded on human nature and experience alone . . . not on any dogma or sacred text", and they welcome atheists and agnostics as well as groups such as the Balcombe Secular Association and the Isle of Man Freethinkers. The "Brights", an internet-based community for all those who take a naturalistic world-view, and whose leading members are Dawkins and Dennett, is broader still, welcoming ethical culturalists, Unitarians and Wiccans. Dennett tells me he's even heard of a group called Atheists for Jesus.

What unites the New Godless, however, is the idea that there is no reason to believe in any supernatural being, and that the absence of any such entity is no bar to humanity constructing its own systems of morals and ethics. For their armoury they do not rely on the theological arguments which exercised Aquinas and Augustine. Facts and scientific method are their weapons.

Dennett jokes that Dawkins, the Charles Simonyi Professor of the Public Understanding of Science at Oxford University, is the "bad cop" and that he's the "good cop". "I made a very deliberate decision to write the book so that religious people could read it," he tells me when we meet at a hotel in St James's. "I didn't want to give them an excuse not to read it. Richard gives them that excuse - he's so hostile and aggressive that he's unsympathetic."

Judging by the reviews, it's not clear Dennett has succeeded. His attempt to break the spell of "thou shalt not study religion intensively" has been taken as aggressive, which annoys him. "I am much more eager to engage in dialogue," he says, "but the reviewers just conveniently lump us together and treat me as indistinguishable from Dawkins. It's hard to combat, but I've got to try."

Whereas Dawkins likens religion to the belief that there are fairies at the bottom of the garden, Dennett declares his hunch to be that "religions that can flourish in conditions of knowledge are fine". But this is as far as his esteem goes. Religion may have positive aspects, he says,"but then so does the Mafia. It keeps neighbourhoods quite secure; there's a very low petty crime rate if the Mafia's in control. That doesn't make it a good thing.

Dennett understands the pull of religion. "Say you're working at the gas station at the crossroads, and you're wondering where the meaning is in this life. Religion, or animal activism, or New Age silliness of one sort or another, offers you a theme." Yet he warns that religion is "the nuclear weapon of rational discussion if, whenever it gets tough, you draw the blinds and play the faith card. It turns it into a sham." And this, he says, is the single most disturbing thing about it.

It's the harder, "higher-tension" religions that really worry Dennett and his non-theistic allies. For example, Buddhist doctrines of reincarnation are, to Dennett, just "really silly". One ally, Andrew Copson of the British Humanist Association, is dismissive of the vogue for New Age thinking and the popular-ity of vaguely "spiritual" schools. "We don't have much time to dedicate to crystals and ear candles," he says. "These things are trends. If anything, they're an argument against the truth of religions, because they don't last long. What happens if you believe that aliens are coming in ten years, and then they don't?"

Copson's association has about 6,000 members, but it claims as kindred spirits, at the least, the many people of no religion who do not specifically identify themselves as humanists. These people need a voice, he argues, because of the continuing prevalence of the notion that religions have a superior morality. "Charles Clarke [the Home Secretary] gave a speech the other day saying that faith gives people values," Copson says. "There's an attempt to use faith as an instrument of cohesion. But other people are not valueless."

The philosopher A C Grayling, a supporter of the BHA, calls for "absolute clarity" in this area. "Very non-rigorous, very confused ideas [about belief] are a source of potential danger," he says. There are, he says, "two monoliths - one is modernity; the other is a very deep and rather simple commitment of faith. These are the two icebergs grinding against one another."

The charge levelled at the New Godless is that, with their rigorous reasoning, testing and experimentation, they are making a religion out of the scientific method. "It's an all-purpose, wild-card smear," retorts Dennett. "It's the last refuge of the sceptic. When someone puts forward a scientific theory that they really don't like, they just try to discredit it as 'scientism'. But when it comes to facts, and explanations of facts, science is the only game in town."

Tested facts are the real ammunition of the New Godless, facts that are undisputed truths. Raising unfounded doubts about those, says Copson, is "a failure to reason properly. The worst thing you can hear is, 'Well, it's my truth.'" Richard Dawkins, naturally, has little time for such viewpoints. "There's this thing called being so open-minded your brains drop out," he says.

The New Godless are roused and, armed with their facts - the scientifically proven truth - they are on the march.