This type of incident is now common. Increasingly, people find it difficult to communicate with their co-religionists. The divide is as great as that between religious and secular people. Many of the faithful feel threatened by those who interpret their tradition differently; it seems their sacred values are in jeopardy. An apparently impassable gulf yawns between liberal and fundamentalist Christians, reform and orthodox Jews, traditional and extremist Muslims. Because of our preoccupation with the so-called clash of civilisations, this internal tension is often overlooked.

It is a year since the London bombings, an act committed in the name of Islam by a viciously disaffected minority, but which violated the essential principles of any religion. Doubtless with this anniversary in mind, the prime minister has complained that British Muslims are not doing enough to deal with the extremists. The "moderate" Muslims, he said testily, must confront the Islamists; they cannot condemn their methods while tacitly condoning their anger. The extremists' anti-western views are wrong, and mainstream Muslims must tell them that violent jihad "is not the religion of Islam".

This regrettable step will put yet more pressure on a community already under strain. It ignores the fact that the chief problem for most Muslims is not "the west" per se, but the suffering of Muslims in Guantánamo, Abu Ghraib, Iraq and Palestine. Many Britons share this dismay, but the strong emphasis placed by Islam upon justice and community solidarity makes this a religious issue for Muslims. When they see their brothers and sisters systematically oppressed and humiliated, some feel as wounded as a Christian who sees the Bible spat upon or the eucharistic host violated.

It is disingenuous of Tony Blair to separate the rising tide of "Islamism" from his unpopular foreign policy, particularly when Palestinians are being subjected to new dangers in Gaza. He is also mistaken to imagine that law-abiding Muslims could bring the extremists to heel in the same way that he disciplines recalcitrant members of his cabinet. This is just not how religious groups operate.

During the 20th century, a militant piety erupted in almost every major world faith: in Buddhism, Sikhism, Hinduism and Confucianism, as well as in Judaism, Christianity and Islam. It is often called "fundamentalism". Its aim is to bring God and/or religion from the sidelines back to centre stage, though very few fundamentalists commit acts of violence. Coined by American Protestants who wanted a return to Christian "fundamentals", the term is unsatisfactory, not least because it suggests a conservative and backward-looking religiosity. In fact, fundamentalists are rebels who have separated themselves irrevocably and on principle from the main body of the faithful. Fundamentalist movements are nearly always the result of an internal dispute with traditional or liberal co-religionists; fundamentalists regard them as traitors who have made too many concessions to modernity. They withdraw from mainstream religious life to create separatist churches, colleges, study groups, madrasas, yeshivas and training camps. Only later, if at all, do fundamentalists turn their wrath against a foreign foe.

Thus Sayyid Qutb (1906-66), whose ideology is followed by most Sunni fundamentalists, had no love for the west, but his jihad was primarily directed against such Muslim rulers as Jamal Abdul Nasser. In order to replace secularist Fatah, Hamas began by attacking the PLO, and was initially funded by Israel in order to undermine Arafat. Osama bin Laden began by campaigning against the Saudi royal family and secularist rulers such as Saddam Hussein; later, when he discovered the extent of their support for these regimes, he declared war against the US. Even when fundamentalists are engaged in a struggle with an external enemy, this internal hostility remains a potent force.

It is unrealistic to hope that radical Islamists will be chastened by a rebuke from "moderate" imams; they have nothing but contempt for traditional Muslims, who they see as part of the problem. Nor are extremists likely to be dismayed when told that terrorism violates the religion of Islam. We often use the word "fundamentalist" wrongly, as a synonym for "orthodox". In fact, fundamentalists are unorthodox - even anti-orthodox. They may invoke the past, but these are innovative movements that promote entirely new doctrines.

Fundamentalist Christians who claim that every word of the Bible is literally true are reading in an essentially modern way; before the advent of our scientifically oriented culture, Jews, Christians and Muslims all relished highly allegorical interpretations of their holy texts. Religious Zionists who regard Israel as sacred also fly in the face of tradition. A hundred years ago, most orthodox rabbis condemned the idea of a Jewish secular state in the Holy Land. In making the assertion that a cleric should be head of state, Ayatollah Khomeini flouted centuries of Shia orthodoxy, which separated religion and politics as a matter of sacred principle.

The same is true of the new emphasis on violent jihad. Until recently, no Muslim thinker had ever claimed it was the central tenet of Islam. The first to make this controversial, even heretical, claim was the Pakistani ideologue Abu Ala Mawdudi in 1939. Like Qutb, he was well aware that this innovation could only be justified by the godless cruelty of modernity. Informed extremists today do not need to be told that their holy war is unorthodox; they already know.

The extremists believe that mainstream Muslims have failed to respond to the current crisis and are proud of their own deviance. Attempting to shift the blame to the already beleaguered Muslim community could further alienate the disaffected. It will certainly not prevent another London bombing.

· Karen Armstrong is the author of The Battle for God, A History of Fundamentalism

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