Perhaps the most admirable part of the response to the conflict that began on Sept. 11 has been a general reluctance to call it a religious war. Officials and commentators have rightly stressed that this is not a battle between the Muslim world and the West, that the murderers are not representative of Islam. President Bush went to the Islamic Center in Washington to reinforce the point. At prayer meetings across the United States and throughout the world, Muslim leaders have been included alongside Christians, Jews and Buddhists.

The only problem with this otherwise laudable effort is that it doesn't hold up under inspection. The religious dimension of this conflict is central to its meaning. The words of Osama bin Laden are saturated with religious argument and theological language. Whatever else the Taliban regime is in Afghanistan, it is fanatically religious. Although some Muslim leaders have criticized the terrorists, and even Saudi Arabia's rulers have distanced themselves from the militants, other Muslims in the Middle East and elsewhere have not denounced these acts, have been conspicuously silent or have indeed celebrated them. The terrorists' strain of Islam is clearly not shared by most Muslims and is deeply unrepresentative of Islam's glorious, civilized and peaceful past. But it surely represents a part of Islam -- a radical, fundamentalist part -- that simply cannot be ignored or denied.

In that sense, this surely is a religious war -- but not of Islam versus Christianity and Judaism. Rather, it is a war of fundamentalism against faiths of all kinds that are at peace with freedom and modernity. This war even has far gentler echoes in America's own religious conflicts -- between newer, more virulent strands of Christian fundamentalism and mainstream Protestantism and Catholicism. These conflicts have ancient roots, but they seem to be gaining new force as modernity spreads and deepens. They are our new wars of religion -- and their victims are in all likelihood going to mount with each passing year.

Osama bin Laden himself couldn't be clearer about the religious underpinnings of his campaign of terror. In 1998, he told his followers, ''The call to wage war against America was made because America has spearheaded the crusade against the Islamic nation, sending tens of thousands of its troops to the land of the two holy mosques over and above its meddling in its affairs and its politics and its support of the oppressive, corrupt and tyrannical regime that is in control.'' Notice the use of the word ''crusade,'' an explicitly religious term, and one that simply ignores the fact that the last few major American interventions abroad -- in Kuwait, Somalia and the Balkans -- were all conducted in defense of Muslims.

Notice also that as bin Laden understands it, the ''crusade'' America is alleged to be leading is not against Arabs but against the Islamic nation, which spans many ethnicities. This nation knows no nation-states as they actually exist in the region -- which is why this form of Islamic fundamentalism is also so worrying to the rulers of many Middle Eastern states. Notice also that bin Laden's beef is with American troops defiling the land of Saudi Arabia -- the land of the two holy mosques,'' in Mecca and Medina. In 1998, he also told followers that his terrorism was ''of the commendable kind, for it is directed at the tyrants and the aggressors and the enemies of Allah.'' He has a litany of grievances against Israel as well, but his concerns are not primarily territorial or procedural. ''Our religion is under attack,'' he said baldly. The attackers are Christians and Jews. When asked to sum up his message to the people of the West, bin Laden couldn't have been clearer: ''Our call is the call of Islam that was revealed to Muhammad. It is a call to all mankind. We have been entrusted with good cause to follow in the footsteps of the messenger and to communicate his message to all nations.''

This is a religious war against ''unbelief and unbelievers,'' in bin Laden's words. Are these cynical words designed merely to use Islam for nefarious ends? We cannot know the precise motives of bin Laden, but we can know that he would not use these words if he did not think they had salience among the people he wishes to inspire and provoke. This form of Islam is not restricted to bin Laden alone.

Its roots lie in an extreme and violent strain in Islam that emerged in the 18th century in opposition to what was seen by some Muslims as Ottoman decadence but has gained greater strength in the 20th. For the past two decades, this form of Islamic fundamentalism has racked the Middle East. It has targeted almost every regime in the region and, as it failed to make progress, has extended its hostility into the West. From the assassination of Anwar Sadat to the fatwa against Salman Rushdie to the decadelong campaign of bin Laden to the destruction of ancient Buddhist statues and the hideous persecution of women and homosexuals by the Taliban to the World Trade Center massacre, there is a single line. That line is a fundamentalist, religious one. And it is an Islamic one.

Most interpreters of the Koran find no arguments in it for the murder of innocents. But it would be naive to ignore in Islam a deep thread of intolerance toward unbelievers, especially if those unbelievers are believed to be a threat to the Islamic world. There are many passages in the Koran urging mercy toward others, tolerance, respect for life and so on. But there are also passages as violent as this: ''And when the sacred months are passed, kill those who join other gods with God wherever ye shall find them; and seize them, besiege them, and lay wait for them with every kind of ambush.'' And this: ''Believers! Wage war against such of the infidels as are your neighbors, and let them find you rigorous.'' Bernard Lewis, the great scholar of Islam, writes of the dissonance within Islam: ''There is something in the religious culture of Islam which inspired, in even the humblest peasant or peddler, a dignity and a courtesy toward others never exceeded and rarely equaled in other civilizations. And yet, in moments of upheaval and disruption, when the deeper passions are stirred, this dignity and courtesy toward others can give way to an explosive mixture of rage and hatred which impels even the government of an ancient and civilized country -- even the spokesman of a great spiritual and ethical religion -- to espouse kidnapping and assassination, and try to find, in the life of their prophet, approval and indeed precedent for such actions.'' Since Muhammad was, unlike many other religious leaders, not simply a sage or a prophet but a ruler in his own right, this exploitation of his politics is not as great a stretch as some would argue.

This use of religion for extreme repression, and even terror, is not of course restricted to Islam. For most of its history, Christianity has had a worse record. From the Crusades to the Inquisition to the bloody religious wars of the 16th and 17th centuries, Europe saw far more blood spilled for religion's sake than the Muslim world did. And given how expressly nonviolent the teachings of the Gospels are, the perversion of Christianity in this respect was arguably greater than bin Laden's selective use of Islam. But it is there nonetheless. It seems almost as if there is something inherent in religious monotheism that lends itself to this kind of terrorist temptation. And our bland attempts to ignore this -- to speak of this violence as if it did not have religious roots -- is some kind of denial. We don't want to denigrate religion as such, and so we deny that religion is at the heart of this. But we would understand this conflict better, perhaps, if we first acknowledged that religion is responsible in some way, and then figured out how and why.

The first mistake is surely to condescend to fundamentalism. We may disagree with it, but it has attracted millions of adherents for centuries, and for a good reason. It elevates and comforts. It provides a sense of meaning and direction to those lost in a disorienting world. The blind recourse to texts embraced as literal truth, the injunction to follow the commandments of God before anything else, the subjugation of reason and judgment and even conscience to the dictates of dogma: these can be exhilarating and transformative. They have led human beings to perform extraordinary acts of both good and evil. And they have an internal logic to them. If you believe that there is an eternal afterlife and that endless indescribable torture awaits those who disobey God's law, then it requires no huge stretch of imagination to make sure that you not only conform to each diktat but that you also encourage and, if necessary, coerce others to do the same. The logic behind this is impeccable. Sin begets sin. The sin of others can corrupt you as well. The only solution is to construct a world in which such sin is outlawed and punished and constantly purged -- by force if necessary. It is not crazy to act this way if you believe these things strongly enough. In some ways, it's crazier to believe these things and not act this way.

In a world of absolute truth, in matters graver than life and death, there is no room for dissent and no room for theological doubt. Hence the reliance on literal interpretations of texts -- because interpretation can lead to error, and error can lead to damnation. Hence also the ancient Catholic insistence on absolute church authority. Without infallibility, there can be no guarantee of truth. Without such a guarantee, confusion can lead to hell.

Dostoyevsky's Grand Inquisitor makes the case perhaps as well as anyone. In the story told by Ivan Karamazov in ''The Brothers Karamazov,'' Jesus returns to earth during the Spanish Inquisition. On a day when hundreds have been burned at the stake for heresy, Jesus performs miracles. Alarmed, the Inquisitor arrests Jesus and imprisons him with the intent of burning him at the stake as well. What follows is a conversation between the Inquisitor and Jesus. Except it isn't a conversation because Jesus says nothing. It is really a dialogue between two modes of religion, an exploration of the tension between the extraordinary, transcendent claims of religion and human beings' inability to live up to them, or even fully believe them.

According to the Inquisitor, Jesus' crime was revealing that salvation was possible but still allowing humans the freedom to refuse it. And this, to the Inquisitor, was a form of cruelty. When the truth involves the most important things imaginable -- the meaning of life, the fate of one's eternal soul, the difference between good and evil -- it is not enough to premise it on the capacity of human choice. That is too great a burden. Choice leads to unbelief or distraction or negligence or despair. What human beings really need is the certainty of truth, and they need to see it reflected in everything around them -- in the cultures in which they live, enveloping them in a seamless fabric of faith that helps them resist the terror of choice and the abyss of unbelief. This need is what the Inquisitor calls the ''fundamental secret of human nature.'' He explains: ''These pitiful creatures are concerned not only to find what one or the other can worship, but to find something that all would believe in and worship; what is essential is that all may be together in it. This craving for community of worship is the chief misery of every man individually and of all humanity since the beginning of time.''

This is the voice of fundamentalism. Faith cannot exist alone in a single person. Indeed, faith needs others for it to survive -- and the more complete the culture of faith, the wider it is, and the more total its infiltration of the world, the better. It is hard for us to wrap our minds around this today, but it is quite clear from the accounts of the Inquisition and, indeed, of the religious wars that continued to rage in Europe for nearly three centuries, that many of the fanatics who burned human beings at the stake were acting out of what they genuinely thought were the best interests of the victims. With the power of the state, they used fire, as opposed to simple execution, because it was thought to be spiritually cleansing. A few minutes of hideous torture on earth were deemed a small price to pay for helping such souls avoid eternal torture in the afterlife. Moreover, the example of such government-sponsored executions helped create a culture in which certain truths were reinforced and in which it was easier for more weak people to find faith. The burden of this duty to uphold the faith lay on the men required to torture, persecute and murder the unfaithful. And many of them believed, as no doubt some Islamic fundamentalists believe, that they were acting out of mercy and godliness.

This is the authentic voice of the Taliban. It also finds itself replicated in secular form. What, after all, were the totalitarian societies of Nazi Germany or Soviet Russia if not an exact replica of this kind of fusion of politics and ultimate meaning? Under Lenin's and Stalin's rules, the imminence of salvation through revolutionary consciousness was in perpetual danger of being undermined by those too weak to have faith -- the bourgeois or the kulaks or the intellectuals. So they had to be liquidated or purged. Similarly, it is easy for us to dismiss the Nazis as evil, as they surely were. It is harder for us to understand that in some twisted fashion, they truly believed that they were creating a new dawn for humanity, a place where all the doubts that freedom brings could be dispelled in a rapture of racial purity and destiny. Hence the destruction of all dissidents and the Jews -- carried out by fire as the Inquisitors had before, an act of purification different merely in its scale, efficiency and Godlessness.

Perhaps the most important thing for us to realize today is that the defeat of each of these fundamentalisms required a long and arduous effort. The conflict with Islamic fundamentalism is likely to take as long. For unlike Europe's religious wars, which taught Christians the futility of fighting to the death over something beyond human understanding and so immune to any definitive resolution, there has been no such educative conflict in the Muslim world. Only Iran and Afghanistan have experienced the full horror of revolutionary fundamentalism, and only Iran has so far seen reason to moderate to some extent. From everything we see, the lessons Europe learned in its bloody history have yet to be absorbed within the Muslim world. There, as in 16th-century Europe, the promise of purity and salvation seems far more enticing than the mundane allure of mere peace. That means that we are not at the end of this conflict but in its very early stages.

America is not a neophyte in this struggle. the United States has seen several waves of religious fervor since its founding. But American evangelicalism has always kept its distance from governmental power. The Christian separation between what is God's and what is Caesar's -- drawn from the Gospels -- helped restrain the fundamentalist temptation. The last few decades have proved an exception, however. As modernity advanced, and the certitudes of fundamentalist faith seemed mocked by an increasingly liberal society, evangelicals mobilized and entered politics. Their faith sharpened, their zeal intensified, the temptation to fuse political and religious authority beckoned more insistently.

Mercifully, violence has not been a significant feature of this trend -- but it has not been absent. The murders of abortion providers show what such zeal can lead to. And indeed, if people truly believe that abortion is the same as mass murder, then you can see the awful logic of the terrorism it has spawned. This is the same logic as bin Laden's. If faith is that strong, and it dictates a choice between action or eternal damnation, then violence can easily be justified. In retrospect, we should be amazed not that violence has occurred -- but that it hasn't occurred more often.

The critical link between Western and Middle Eastern fundamentalism is surely the pace of social change. If you take your beliefs from books written more than a thousand years ago, and you believe in these texts literally, then the appearance of the modern world must truly terrify. If you believe that women should be consigned to polygamous, concealed servitude, then Manhattan must appear like Gomorrah. If you believe that homosexuality is a crime punishable by death, as both fundamentalist Islam and the Bible dictate, then a world of same-sex marriage is surely Sodom. It is not a big step to argue that such centers of evil should be destroyed or undermined, as bin Laden does, or to believe that their destruction is somehow a consequence of their sin, as Jerry Falwell argued. Look again at Falwell's now infamous words in the wake of Sept. 11: ''I really believe that the pagans, and the abortionists, and the feminists, and the gays and lesbians who are actively trying to make that an alternative lifestyle, the A.C.L.U., People for the American Way -- all of them who have tried to secularize America -- I point the finger in their face and say, 'You helped this happen.'''

And why wouldn't he believe that? He has subsequently apologized for the insensitivity of the remark but not for its theological underpinning. He cannot repudiate the theology -- because it is the essence of what he believes in and must believe in for his faith to remain alive.

The other critical aspect of this kind of faith is insecurity. American fundamentalists know they are losing the culture war. They are terrified of failure and of the Godless world they believe is about to engulf or crush them. They speak and think defensively. They talk about renewal, but in their private discourse they expect damnation for an America that has lost sight of the fundamentalist notion of God.

Similarly, Muslims know that the era of Islam's imperial triumph has long since gone. For many centuries, the civilization of Islam was the center of the world. It eclipsed Europe in the Dark Ages, fostered great learning and expanded territorially well into Europe and Asia. But it has all been downhill from there. From the collapse of the Ottoman Empire onward, it has been on the losing side of history. The response to this has been an intermittent flirtation with Westernization but far more emphatically a reaffirmation of the most irredentist and extreme forms of the culture under threat. Hence the odd phenomenon of Islamic extremism beginning in earnest only in the last 200 years.

With Islam, this has worse implications than for other cultures that have had rises and falls. For Islam's religious tolerance has always been premised on its own power. It was tolerant when it controlled the territory and called the shots. When it lost territory and saw itself eclipsed by the West in power and civilization, tolerance evaporated. To cite Lewis again on Islam: ''What is truly evil and unacceptable is the domination of infidels over true believers. For true believers to rule misbelievers is proper and natural, since this provides for the maintenance of the holy law and gives the misbelievers both the opportunity and the incentive to embrace the true faith. But for misbelievers to rule over true believers is blasphemous and unnatural, since it leads to the corruption of religion and morality in society and to the flouting or even the abrogation of God's law.''

Thus the horror at the establishment of the State of Israel, an infidel country in Muslim lands, a bitter reminder of the eclipse of Islam in the modern world. Thus also the revulsion at American bases in Saudi Arabia. While colonialism of different degrees is merely political oppression for some cultures, for Islam it was far worse. It was blasphemy that had to be avenged and countered.

I cannot help thinking of this defensiveness when I read stories of the suicide bombers sitting poolside in Florida or racking up a $48 vodka tab in an American restaurant. We tend to think that this assimilation into the West might bring Islamic fundamentalists around somewhat, temper their zeal. But in fact, the opposite is the case. The temptation of American and Western culture -- indeed, the very allure of such culture -- may well require a repression all the more brutal if it is to be overcome. The transmission of American culture into the heart of what bin Laden calls the Islamic nation requires only two responses -- capitulation to unbelief or a radical strike against it. There is little room in the fundamentalist psyche for a moderate accommodation. The very psychological dynamics that lead repressed homosexuals to be viciously homophobic or that entice sexually tempted preachers to inveigh against immorality are the very dynamics that lead vodka-drinking fundamentalists to steer planes into buildings. It is not designed to achieve anything, construct anything, argue anything. It is a violent acting out of internal conflict.

And America is the perfect arena for such acting out. For the question of religious fundamentalism was not only familiar to the founding fathers. In many ways, it was the central question that led to America's existence. The first American immigrants, after all, were refugees from the religious wars that engulfed England and that intensified under England's Taliban, Oliver Cromwell. One central influence on the founders' political thought was John Locke, the English liberal who wrote the now famous ''Letter on Toleration.'' In it, Locke argued that true salvation could not be a result of coercion, that faith had to be freely chosen to be genuine and that any other interpretation was counter to the Gospels. Following Locke, the founders established as a central element of the new American order a stark separation of church and state, ensuring that no single religion could use political means to enforce its own orthodoxies.

We cite this as a platitude today without absorbing or even realizing its radical nature in human history -- and the deep human predicament it was designed to solve. It was an attempt to answer the eternal human question of how to pursue the goal of religious salvation for ourselves and others and yet also maintain civil peace. What the founders and Locke were saying was that the ultimate claims of religion should simply not be allowed to interfere with political and religious freedom. They did this to preserve peace above all -- but also to preserve true religion itself.

The security against an American Taliban is therefore relatively simple: it's the Constitution. And the surprising consequence of this separation is not that it led to a collapse of religious faith in America -- as weak human beings found themselves unable to believe without social and political reinforcement -- but that it led to one of the most vibrantly religious civil societies on earth. No other country has achieved this. And it is this achievement that the Taliban and bin Laden have now decided to challenge. It is a living, tangible rebuke to everything they believe in.

That is why this coming conflict is indeed as momentous and as grave as the last major conflicts, against Nazism and Communism, and why it is not hyperbole to see it in these epic terms. What is at stake is yet another battle against a religion that is succumbing to the temptation Jesus refused in the desert -- to rule by force. The difference is that this conflict is against a more formidable enemy than Nazism or Communism. The secular totalitarianisms of the 20th century were, in President Bush's memorable words, ''discarded lies.'' They were fundamentalisms built on the very weak intellectual conceits of a master race and a Communist revolution.

But Islamic fundamentalism is based on a glorious civilization and a great faith. It can harness and co-opt and corrupt true and good believers if it has a propitious and toxic enough environment. It has a more powerful logic than either Stalin's or Hitler's Godless ideology, and it can serve as a focal point for all the other societies in the world, whose resentment of Western success and civilization comes more easily than the arduous task of accommodation to modernity. We have to somehow defeat this without defeating or even opposing a great religion that is nonetheless extremely inexperienced in the toleration of other ascendant and more powerful faiths. It is hard to underestimate the extreme delicacy and difficulty of this task.

In this sense, the symbol of this conflict should not be Old Glory, however stirring it is. What is really at issue here is the simple but immensely difficult principle of the separation of politics and religion. We are fighting not for our country as such or for our flag. We are fighting for the universal principles of our Constitution -- and the possibility of free religious faith it guarantees. We are fighting for religion against one of the deepest strains in religion there is. And not only our lives but our souls are at stake.