The novels of Graham Greene are full of reluctant Christians, men and women who would like to be rid of God but find themselves stuck with him like some lethal addiction. There are, however, reluctant atheists as well, people who long to dunk themselves in the baptismal font but can't quite bring themselves to believe. George Steiner and Roger Scruton have both been among this company at various stages of their careers. The agnostic philosopher Simon Critchley, who currently has a book in the press entitled The Faith of the Faithless, is one of a whole set of leftist thinkers today (Slavoj Žižek, Alain Badiou, Giorgio Agamben) whose work draws deeply on Christian theology. In this respect, the only thing that distinguishes them from the Pope is that they don't believe in God. It is rather like coming across a banker who doesn't believe in profit.

Such reluctant non-belief goes back a long way. Machiavelli thought religious ideas, however vacuous, were a useful way of terrorising the mob. Voltaire rejected the God of Christianity, but was anxious not to infect his servants with his own scepticism. Atheism was fine for the elite, but might breed dissent among the masses. The 18th-century Irish philosopher John Toland, who was rumoured to be the bastard son of a prostitute and a spoilt priest, clung to a "rational" religion himself, but thought the rabble should stick with their superstitions. There was one God for the rich and another for the poor. Edward Gibbon, one of the most notorious sceptics of all time, held that the religious doctrines he despised could still be socially useful. So does the German philosopher Jurgen Habermas today.

Diderot, a doyen of the French Enlightenment, wrote that the Christian gospel might have been a less gloomy affair if Jesus had fondled the breasts of the bridesmaids at Cana and caressed the buttocks of St John. Yet he, too, believed that religion was essential for social unity. Matthew Arnold feared the spread of godlessness among the Victorian working class. It could be countered, he thought, with a poeticised form of a Christianity in which he himself had long ceased to believe. The 19th-century French philosopher Auguste Comte, an out-and-out materialist, designed an ideal society complete with secular versions of God, priests, sacraments, prayer and feast days.

There is something deeply disingenuous about this whole tradition. "I don't believe myself, but it is politically prudent that you should" is the slogan of thinkers supposedly devoted to the integrity of the intellect. If the Almighty goes out of the window, how are social order and moral self-discipline to be maintained? It took the barefaced audacity of Friedrich Nietzsche to point out that if God was dead, then so was Man – or at least the conception of humanity favoured by the guardians of social order. The problem was not so much that God had inconveniently expired; it was that men and women were cravenly pretending that he was still alive, and thus refusing to revolutionise their idea of themselves.

God may be dead, but Alain de Botton's Religion for Atheists is a sign that the tradition from Voltaire to Arnold lives on. The book assumes that religious beliefs are a lot of nonsense, but that they remain indispensible to civilised existence. One wonders how this impeccably liberal author would react to being told that free speech and civil rights were all bunkum, but that they had their social uses and so shouldn't be knocked. Perhaps he might have the faintest sense of being patronised. De Botton claims that one can be an atheist while still finding religion "sporadically useful, interesting and consoling", which makes it sound rather like knocking up a bookcase when you are feeling a bit low. Since Christianity requires one, if need be, to lay down one's life for a stranger, he must have a strange idea of consolation. Like many an atheist, his theology is rather conservative and old-fashioned.

De Botton does not want people literally to believe, but he remains a latter-day Matthew Arnold, as his high Victorian language makes plain. Religion "teaches us to be polite, to honour one another, to be faithful and sober", as well as instructing us in "the charms of community". It all sounds tediously neat and civilised. This is not quite the gospel of a preacher who was tortured and executed for speaking up for justice, and who warned his comrades that if they followed his example they would meet with the same fate. In De Botton's well-manicured hands, this bloody business becomes a soothing form of spiritual therapy, able to "promote morality (and) engender a spirit of community". It is really a version of the Big Society.

Like Comte, De Botton believes in the need for a host of "consoling, subtle or just charming rituals" to restore a sense of community in a fractured society. He even envisages a new kind of restaurant in which strangers would be forced to sit together and open up their hearts to one another. There would be a Book of Agape on hand, which would instruct diners to speak to each other for prescribed lengths of time on prescribed topics. Quite how this will prevent looting and rioting is not entirely clear.

In Comtist style, De Botton also advocates secular versions of such sacred events as the Jewish Day of Atonement, the Catholic Mass and the Zen Buddhist tea ceremony. It is surprising he does not add Celtic versus Rangers. He is also keen on erecting billboards that carry moral or spiritual rather than commercial messages, perhaps (one speculates) in the style of "Leave Young Ladies Alone" or "Tortoises Have Feelings As Well". It is an oddly Orwellian vision for a self-proclaimed libertarian. Religious faith is reduced to a set of banal moral tags. We are invited to contemplate St Joseph in order to learn "how to face the trials of the workplace with a modest and uncomplaining temper". Not even the Walmart management have thought of that one. As a role model for resplendent virtue, we are offered not St Francis of Assisi but Warren Buffett.

What the book does, in short, is hijack other people's beliefs, empty them of content and redeploy them in the name of moral order, social consensus and aesthetic pleasure. It is an astonishingly impudent enterprise. It is also strikingly unoriginal. Liberal-capitalist societies, being by their nature divided, contentious places, are forever in search of a judicious dose of communitarianism to pin themselves together, and a secularised religion has long been one bogus solution on offer. The late Christopher Hitchens, who some people think is now discovering that his broadside God Is Not Great was slightly off the mark, would have scorned any such project. He did not consider that religion was a convenient fiction. He thought it was disgusting. Now there's something believers can get their teeth into …

• Terry Eagleton's Why Marx Was Right is published by Yale.