When I taught Religious Studies at Georgia State University, I used to assign Nazi theology and biblical arguments for slavery. I wanted my students to understand how Christianity has been used to justify what they knew to be horrors. Their initial reaction was almost always to protest that anyone who could enslave another human being or justify murdering Jews was not a real Christian. The intent of such protests—withholding the imprimatur of Christianity from evildoers—was laudable, but this stance also had the effect of insulating Christianity from criticism. Christians who owned slaves or ran concentration camps were not really Christians at all.

Since 9/11, many Americans, most notably President George W. Bush, have made a similar move concerning Islam and terrorism: If Islam is, as the president has repeatedly said, a "religion of peace," those who committed the atrocities of 9/11 cannot have been real Muslims. Muslims, it seems, can do no wrong; the Muslims who flew airplanes into the World Trade Center were not really Muslims at all.

The root meaning of the term Islam is submission, and classically a Muslim is anyone who submits to God by uttering with faith, "There is no God but Allah, and Muhammad is the messenger of God." Over time, however, many Muslims have developed far more stringent qualifications for the category of real Muslim. Some Sunni Muslims see all Shiite Muslims as pretenders to the true faith. Some Shiites view Sunnis likewise. And some Muslims have even stricter qualifications for their fellow travelers. Members of al–Qaeda, for example, believe that any so–called Muslim who holds views about Islam different from their own is not a real Muslim.

The Islamic tradition has a word—takfir—for this sort of exclusivist thinking, which goes all the way back to the first few decades after Muhammad's death. And what takfir amounts to is excommunication by imagination.

To outsiders, it might appear that such debates about who is and who is not a true Muslim are nothing more than intramural skirmishes. Why should American Christians, for example, care about how Muslims in Pakistan or the Middle East patrol the border zones of their faith? But the implications of this ongoing war of words are immense, since it is the idea of takfir that opened the Islamic tradition to the horrors of 9/11.

The Looming Tower, a masterful narrative history by New Yorker writer Lawrence Wright, traces the steps and missteps—in the United States, Saudi Arabia, and elsewhere—that brought us 9/11. Wright focuses on four people—Osama bin Laden, the Egyptian intellectual Ayman al–Zawahiri, a Saudi intelligence officer named Prince Turki al–Faisal, and an FBI counterterrorism agent named John O'Neill.

Along the way he offers his readers a series of telling details about what is arguably the event of this generation. Here readers will learn that bin Laden's father and brother died in separate plane crashes, and that bin Laden (like President Bush?) watched Bonanza as a child. They will also learn that at one point, bin Laden attempted to downshift al–Qaeda into an agricultural organization, and that this organization's first traitor used some of his $1 million reward money to buy a winning ticket to the New Jersey lottery. Finally, and most eerily, they will learn that, after being edged out of the FBI in August 2001 for (among other things) his obsession with al–Qaeda, the FBI agent John O'Neill began working at the World Trade Center. Only days after he started this new job he would be numbered among the dead.

Wright's book aims to tell a story, and it tells a good one. But his style of storytelling means it is largely up to the reader to suss out the key themes. And for this reader at least the central theme is takfir.

Takfir has a long legacy in Islamic thought, but for the most part Muslim thinkers have followed the medieval scholar al–Ghazali in preferring to greet other Muslims with a spirit of tolerance rather than accusations of unbelief. If a Muslim calls another Muslim a fake, one popular saying goes, it is true about one of them (presumably, the intolerant accuser).

Wright traces the current takfir vogue among Islamists to the Manichaean thinking of the Wahhabis and particularly to Sayyid Qutb, an Egyptian intellectual who far more than bin Laden himself laid the intellectual groundwork for Islamist terrorism. In a chapter on Qutb called "The Martyr," Wright portrays Qutb (who was hanged in Egypt in 1966) as the progenitor of contemporary Islamist politics—for his claims that Islam is at war with modernity, that Muslims today must return to the pure Islam of Muhammad and his companions, and that all subsequent innovations to the tradition (including the Shiite and Sufi traditions) are but bastardizations of the one true faith. What is most chilling about Qutb's thought, however, is not so much his theological primitivism as his insistence that Muslims who disagree with him on any of these key points are apostates. His world is neatly divided into two warring parties—true Islam and barbarism—and any Muslim who disagrees with him is a barbarian. As Wright convincingly argues, it was this intellectual sleight of hand that "would open the door to terror."

To understand why this line of thought matters, it is important to note that the Qur'an plainly forbids the killing of other Muslims. As Wright discusses, the Qur'an does not shrink from war with idolaters. "Slay the idolaters wherever you find them," it reads, and "fight those who do not believe in God." Yet the Islamic tradition, including its four main schools of jurisprudence, also says that women and children must be spared in combat, and that Muslims must not target fellow Muslims for death. Commit the latter crime and you will spend eternity in hell.

In the World Trade Center, of course, many Muslims were murdered. The killings of Muslims in suicide bombings by al–Qaeda operatives and their imitators in Afghanistan and Iraq continue to mount. And it is the notion of takfir that has made all this possible.

The Looming Toweris a groundbreaking book in explaining the rise and success of al–Qaeda. Wright demonstrates convincingly what many Americans had long suspected—that 9/11 might have been prevented if only the various intelligence agencies had learned the basic kindergarten lesson of the importance of sharing with your friends.

For me, however, the point this book really brought home is that what we are witnessing here is not so much a "clash of civilizations" between Islam and the West as a clash of Islamic civilizations—a culture war between, on the one hand, Muslims who believe that the Islamic tradition means what it says when it comes to killing women and children and Muslims and, on the other hand, those who don't—between Muslims who accept at face value the professions of faith of their fellow Muslims and those who excommunicate by imagination anyone who disagrees with their narrow interpretation of the one true faith.

More effectively than any other book I have read, The Looming Tower gives the lie to the idea that there is one Muslim world. It also steers clear of the pious foolishness that no real Muslim could crash a plane into a building of innocents. After all, those who steered those planes into the World Trade Center and the Pentagon were convinced that no real Muslim would refuse such an honor.

Stephen Prothero, chair of the Department of Religion at Boston University, is the author of Religious Literacy: What Every American Needs to Know—And Doesn't, just published by HarperSanFrancisco.

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