I love the remark made by one Oxford don about another: "On the surface, he's profound, but deep down, he's superficial." That sentence has more than once come to mind when reading the new atheists.

Future intellectual historians will look back with wonder at the strange phenomenon of seemingly intelligent secularists in the twenty-first century believing that if they could show that the first chapters of Genesis are not literally true, that the universe is more than 6,000 years old and there might be other explanations for rainbows than as a sign of God's covenant after the flood, the whole of humanity's religious beliefs would come tumbling down like a house of cards and we would be left with a serene world of rational non-believers getting on famously with one another.

Whatever happened to the intellectual depth of the serious atheists - the forcefulness of Hobbes, the passion of Spinoza, the wit of Voltaire, the world-shattering profundity of Nietzsche? Where is there the remotest sense that they have grappled with the real issues, which have nothing to do with science and the literal meaning of scripture and everything to do with the meaningfulness or otherwise of human life, the existence or non-existence of an objective moral order, the truth or falsity of the idea of human freedom, and the ability or inability of society to survive without the rituals, narratives and shared practices that create and sustain the social bond?

A significant area of intellectual discourse - the human condition sub specie aeternitatis - has been dumbed down to the level of a school debating society. Does it matter? Should we not simply accept that just as there are some people who are tone deaf and others who have no sense of humour, so there are some who simply do not understand what is going on in the Book of Psalms, who lack a sense of transcendence or the miracle of being, who fail to understand what it might be to see human life as a drama of love and forgiveness or be moved to pray in penitence or thanksgiving? Some people get religion; others don't. Why not leave it at that?

Fair enough, perhaps, for some. But not for more attuned readers, because religion has social, cultural and political consequences, and you cannot expect the foundations of western civilisation to crumble and leave the rest of the building intact. That is what the greatest of all atheists, Nietzsche, understood with terrifying clarity and what his latter-day successors fail to grasp at all.

Time and again in his later writings, Nietzsche tells us that losing Christian faith will mean abandoning Christian morality. No more "Love your neighbour as yourself'; instead the will to power. No more "Thou shalt not"; instead, people would live by the law of nature, the strong dominating or eliminating the weak. "An act of injury, violence, exploitation or destruction cannot be 'unjust' as such, because life functions essentially in an injurious, violent, exploitative and destructive manner."

Nietzsche was not an anti-Semite, but there are passages in his writing that come close to justifying a Holocaust. This had nothing to do with him personally and everything to do with the logic of Europe losing its Christian ethic. Already in 1843, a year before Nietzsche was born, Heinrich Heine wrote:

"A drama will be enacted in Germany compared to which the French Revolution will seem like a harmless idyll. Christianity restrained the martial ardour of the Germans for a time but it did not destroy it; once the restraining talisman is shattered, savagery will rise again ... the mad fury of the berserk, of which Nordic poets sing and speak."

Nietzsche and Heine were making the same point. Lose the Judeo-Christian sanctity of life and there will be nothing to contain the evil men do when given the chance and the provocation.

Richard Dawkins, whom I respect, partly understands this. He has said often that Darwinism is a science, not an ethic. Turn natural selection into a code of conduct and you get disaster. But if asked where we get our morality from, if not from science or religion, the new atheists start to stammer. They tend to argue that ethics is obvious (which it isn't), or natural (which it manifestly isn't either) and end up vaguely hinting that this isn't their problem. Let someone else worry about it.

The history of Europe since the eighteenth century has been the story of successive attempts to find alternatives to God as an object of worship - among them the nation state, race and the Communist Manifesto. After this cost humanity two world wars, a Cold War and a hundred million lives, we have turned to more pacific forms of idolatry, among them the market, the liberal democratic state and the consumer society, all of which are ways of saying that there is no morality beyond personal choice so long as you do no harm to others.

Even so, the costs are beginning to mount up. Levels of trust have plummeted throughout the West as one group after another - bankers, CEOs, media personalities, parliamentarians, the press - has been hit by scandal. Marriage has all but collapsed as an institution, with 40% of children born outside it and 50% of marriages ending in divorce. Rates of depressive illness and stress-related syndromes have rocketed especially among the young. A recent survey showed that the average 18- to 35-year-old has 237 Facebook friends. When asked how many they could rely on in a crisis, the average answer was two. A quarter said one. An eighth said none.

None of this should surprise us. This is what a society built on materialism, individualism and moral relativism looks like. It maximises personal freedom but at a cost. As Michael Walzer puts it:

"This freedom, energising and exciting as it is, is also profoundly disintegrative, making it very difficult for individuals to find any stable communal support, very difficult for any community to count on the responsible participation of its individual members. It opens solitary men and women to the impact of a lowest common denominator, commercial culture."

In my time as Chief Rabbi, I have seen two highly significant trends. First, parents are more likely than they were to send their children to faith schools. They want their children exposed to a strong substantive ethic of responsibility and restraint. Second, religious people, Jews especially, are more fearful of the future than they were. Our newly polarised culture is far less tolerant than old, mild Christian Britain.

In one respect the new atheists are right. The threat to western freedom in the twenty-first century is not from fascism or communism but from a religious fundamentalism combining hatred of the other, the pursuit of power and contempt for human rights. But the idea that this can be defeated by individualism and relativism is naive almost beyond belief.

Humanity has been here before. The precursors of today's scientific atheists were Epicurus in third-century BCE Greece and Lucretius in first-century Rome. These were two great civilisations on the brink of decline. Having lost their faith, they were no match for what Bertrand Russell calls "nations less civilised than themselves but not so destitute of social cohesion." The barbarians win. They always do.

The new barbarians are the fundamentalists who seek to impose a single truth on a plural world. Though many of them claim to be religious, they are actually devotees of the will to power. Defeating them will take the strongest possible defence of freedom, and strong societies are always moral societies. That does not mean that they need be religious. It is just that, in the words of historian Will Durant, "There is no significant example in history, before our time, of a society successfully maintaining moral life without the aid of religion."

I have no desire to convert others to my religious beliefs. Jews don't do that sort of thing. Nor do I believe that you have to be religious to be moral. But Durant's point is the challenge of our time. I have not yet found a secular ethic capable of sustaining in the long run a society of strong communities and families on the one hand, altruism, virtue, self-restraint, honour, obligation and trust on the other.

A century after a civilisation loses its soul it loses its freedom also. That should concern all of us, believers and non-believers alike.

Lord Jonathan Sacks is the outgoing Chief Rabbi of the United Hebrew Congregations of the Commonwealth. His latest book is The Great Partnership: God, Science and the Search for Meaning. This article was originally published in The Spectator and appears here with permission.