And yet – the believers still claim to know! Not just to know that god exists, and that he created and supervised the whole enterprise, but also to know what "he" demands of us – from our diet to our observances to our sexual morality. In other words, in a vast and complicated discussion where we know more and more about less and less, yet can still hope for some enlightenment as we proceed, one faction – itself composed of mutually warring factions – has the sheer arrogance to tell us that we already have all the essential information we need. Such stupidity, combined with such pride, should be enough on its own to exclude "belief" from the debate. The person who is certain, and who claims divine warrant for his certainty, belongs now to the infancy of our species. It may be a long farewell, but it has begun and, like all farewells, should not be protracted.

A week before the events of September 11, 2001, I was on a panel with Dennis Prager, who is one of America's better-known religious broadcasters. He challenged me in public to answer what he called a "straight yes/no question," and I happily agreed. Very well, he said. I was to imagine myself in a strange city as the evening was coming on. Toward me I was to imagine that I saw a large group of men approaching. Now – would I feel safer, or less safe, if I was to learn that they were just coming from a prayer meeting? As the reader will see, this is not a question to which a yes/no answer can be given. But I was able to answer it as if it were not hypothetical. "Just to stay within the letter 'B,' I have actually had that experience in Belfast, Beirut, Bombay, Belgrade, Bethlehem, and Baghdad. In each case I can say absolutely, and can give my reasons, why I would feel immediately threatened if I thought that the group of men approaching me in the dusk were coming from a religious observance."

On February 14, 1989, my friend Salman Rushdie was hit by a simultaneous death sentence and life sentence, for the crime of writing a work of fiction. To be more precise, the theocratic head of a foreign state – the Ayatollah Khomeini of Iran – publicly offered money, in his own name, to suborn the murder of a novelist who was a citizen of another country. Those who were encouraged to carry out this bribed assassination scheme, which extended to "all those involved in the publication" of The Satanic Verses, were offered not just the cold cash but also a free ticket to paradise. It is impossible to imagine a greater affront to every value of free expression… (*) It put me on notice of what I already knew. It is not possible for me to say, Well, you pursue your Shiite dream of a hidden imam and I pursue my study of Thomas Paine and George Orwell, and the world is big enough for both of us. The true believer cannot rest until the whole world bows the knee.

[After 9/11] I was able to write in a further reply to Dennis Prager, now you have your answer. The nineteen suicide murderers of New York and Washington and Pennsylvania were beyond any doubt the most sincere believers on those planes. Perhaps we can hear a little less about how "people of faith" possess moral advantages that others can only envy.

There may be someone who can explain the sexual and other cruelties of the religious without any reference to the obsession with celibacy, but that someone will not be me. I simply laugh when I read the Koran, with its endless prohibitions on sex and its corrupt promise of infinite debauchery in the life to come… The homicidal lunatics – rehearsing to be genocidal lunatics – of 9/11 were perhaps tempted by virgins, but it is far more revolting to contemplate that, like so many of their fellow jihadists, they were virgins. Like monks of old, the fanatics are taken early from their families, taught to despise their mothers and sisters, and come to adulthood without ever having had a normal conversation, let alone a normal relationship, with a woman. This is disease by definition.

Not only did Islam begin by condemning all doubters to eternal fire, but it still claims the right to so in almost all of its dominions, and still preaches that these same dominions can and must be extended by war… It makes immense claims for itself, invokes prostrate submission or "surrender" as a maxim to its adherents, and demands deference and respect from nonbelievers into the bargain. There is nothing – absolutely nothing – in its teachings that can even begin to justify such arrogance and presumption.

The eighth sura [of the Koran], revealed at Medina, deals at some length with the justified spoils of war and dwells continually on the further postmortem "torments of fire" that await those who are defeated by the believers. It was this very sura that was to be used by Saddam Hussein to justify his mass murder and dispossession of the people of Kurdistan.

In my own recent life in Washington, I have been bombarded with obscene and menacing phone calls from Muslims, promising to punish my family because I do not support a campaign of lies and hatred and violence against democratic Denmark.

As I write, a version of the Inquisition [the Iranian theocracy] is about to lay hands on a nuclear weapon… This puts the confrontation between faith and civilization on a whole new footing. Until relatively recently, those who adopted the clerical path had to pay a heavy price for it. Their societies would decay, their economies would contract, their best minds would go to waste or take themselves elsewhere, and they would consistently be outdone by societies that had learned to tame and sequester the religious impulse. A country like Afghanistan would simply rot. Bad enough as it was, it became worse on September 11, 2001, when from Afghanistan the holy order was given to annex two famous achievements of modernism – the high-rise building and the jet aircraft – and use them for immolation and human sacrifice. The succeeding stage, very plainly announced in hysterical sermons, was to be the moment when apocalyptic nihilists coincided with Armageddon weaponry. Faith-based fanatics could not design anything as useful or beautiful as a skyscarper or a passenger aircraft. But, continuing their long history of plagiarism, they could borrow and steal these things and use them as a negation.

This book has been about the oldest argument in human history, but almost every week that I was engaged in writing it, I was forced to break off and take part in the argument as it was actually continuing. These arguments tended to take ugly forms: I was not so often leaving my desk to go and debate with some skilful old Jesuit at Georgetown, but rather hurrying out to show solidarity at the embassy of Denmark, a small democratic country in northern Europe whose other embassies were going up in smoke because of the appearance of a few caricatures in a newspaper in Copenhagen. (*) This last confrontation was an especially depressing one. Islamic mobs were violating diplomatic immunity and issuing death threats against civilians, yet the response from His Holiness the Pope and the archbishop of Canterbury was to condemn – the cartoons! In my own profession, there was a rush to see who could capitulate the fastest, by reporting on the disputed images without actually showing them. Euphemistic noises were made about the need to show "respect," but I know quite a number of the editors concerned and can say for a certainty that the chief motive for "restraint" was simple fear. In other words, a handful of religious bullies and bigmouths could, so to speak, outvote the tradition of free expression in its Western heartland. And in the year 2006, at that! To the ignoble motive of fear one must add the morally lazy practice of relativism: no group of nonreligious people threatening and practicing violence would have been granted such an easy victory, or had their excuses – not that they offered any of their own – made for them.

Religion even boasts a special branch of itself, devoted to the study of the end. It calls itself "eschatology," and broods incessantly on the passing away of all earthly things. This death cult refuses to abate, even though we have every reason to think that "earthly things" are all that we have, or are ever going to have. Yet in our hands and within our view is a whole universe of discovery and clarification, which is a pleasure to study in itself, gives the average person access to insights that not even Darwin or Einstein possessed, and offers the promise of near-miraculous advances in healing, in energy, and in peaceful exchange between different cultures. Yet millions of people in all societies still prefer the myths of the cave and the tribe and the blood sacrifice… Confronted with undreamed-of vistas inside our own evolving cortex, in the farthest reaches of the known universe, and in the proteins and acids which constitute our nature, religion offers either annihilation in the name of god, or else the false promise that if we take a knife to our foreskins, or pray in the right direction, or ingest pieces of wafer, we shall be "saved."

To all those whom I do not know, and who live in the worlds where superstition and barbarism are still dominant, and into whose hands I hope this little book may fall, I offer the modest encouragement of an older wisdom. It is in fact this, and not any arrogant preaching, that comes to us out of the whirlwind: Die Stimme der Vernunft ist leise. Yes, "The voice of Reason is soft." But it is very peristent. In this, and in the lives and minds of combatants known and unknown, we repose our chief hope.