The return of ideology has taken us all by surprise, because no one expected it all to be about religion. Twenty years ago, when the Berlin Wall fell and it seemed reasonable to suppose that all the big questions about how to organise society had been solved by history, if you had asked what could possibly disrupt this progressive consensus, hardly anyone would have supposed that the answer had anything to do with God.
There may have been a few prescient pessimists who thought Islam would be an important and dangerous disruption on the forward march to the future - perhaps important and dangerous enough to need quelling with a few brisk, punitive expeditions - but even such pessimists could hardly have imagined the fiasco that the wars in Iraq and Afghanistan have turned out to be, nor 9/11 attacks and the widespread fear and loathing they have produced.
Nor could anyone have foreseen the emergence of the religious right as such a dominant force in American politics and its extraordinary takeover of the Republican party.
But now the Reaganite model of capitalism is collapsing around us, liberal democracy is no longer poised to take over the world but worrying about where and how it may survive - and the arguments about religion are back as fiercely as ever, and almost as popular. This is extraordinary. Communism would not have fallen in the way it did were it not for the passionate Roman Catholicism of the Poles, and even in the other countries of eastern Europe there were Christian groupings at the forefront of the revolutions of 1989: Lutherans in Germany, Calvinists in Romania. But these atavistic throwbacks, while they might have outlasted communism, seemed helpless against capitalism.
Sure enough, the Catholic church in Ireland imploded in scandals, and even in Poland lost much of its influence. The slow etiolation of liberal Protestantism continued. The simple message of hedonistic liberalism - there is probably no God; there is nothing to worry about; enjoy yourselves - seemed in the west entirely self-evident. The wierdos who didn't know the enjoyment of this life was all one could hope for were clearly dying off and religion was no more than a "licensed insanity", in the words of John Bowker, then dean of Trinity.
What changed? What brought us to the contemporary world, where millions of Muslims, Christians, atheists and Hindus understand the existence of infidels and heretics as an existential threat, against which almost any degree of violence is sometimes justified? The one thing that almost all intellectuals would now agree on is that other people's theological and philosophical opinions threaten the continuation of human life, a belief unknown since the cold war. Of course, we all disagree about which beliefs are dangerous. Nor is there any accepted way to resolve these disagreements, or even to live with them.
Part of the answer to what changed is obviously Islam. Without the attacks of September 11, and before them the fatwa on Salman Rushdie, the idea that religion might be dangerous as well as wrong would not be nearly so widespread. But it is also, I think, related to a general realisation that, for whatever reason, the welfare states of western Europe have fallen far short of their original promise. Whether you blame Thatcherism or the 60s, it no longer seems self-evident that everyone will continue to get richer, less violent and better educated in the way that everyone took for granted in the years when churches were collapsing.
Marx said: "Religious distress is at the same time the expression of real distress and the protest against real distress. Religion is the sigh of the oppressed creature, the heart of a heartless world, just as it is the spirit of a spiritless situation. It is the opium of the people... The demand to give up the illusion about its condition is the demand to give up a condition that needs illusions."
I agree, but what if we cannot give up the "condition that needs illusions"? What if the demand to give it up is itself as infantile a piece of wish-fulfilment as anything in anyone's scriptures? The fear of these questions is surely part of the explanation for the hysterical and apocalyptic tone of some atheists.
Asking these questions doesn't guarantee an answer, but they suddenly seem pressing, which guarantees that religion will continue its resurrection. That's one reason for the paper's new site, Comment is free: Belief, today.
Andrew Brown, the author of The Darwin Wars and editor of the Guardian's new faith site at guardian.co.uk/commentisfree/belief