What is really going on during religious revivals or in the growth of political movements based on religion, or of terrorist groups claiming religious justification, are vexed issues, but between, say, the Iranian revolution and the emergence of al-Qaida, there have been some clues. Those are both dates of developments within the Islamic world, but this is a period which also encompasses the growth of the Christian right in the US, an increase in religious influence in Israel, a war in the Balkans in which religiously derived identities played a baleful part, the emergence of a more extreme form of Hinduism in India and the sometimes intolerant reassertiveness of the Orthodox churches in former communist states.

Such a list suggests a pathology, and that is certainly there, as the Mombasa outrage confirms. But for the more general phenomena of religious politics it might be better to go back to that understanding of religious change which sees it as reflecting the appearance of new classes, but also as shaped by local political tradition. The Middle East, in particular, shows the range of different kinds of religious change that have accompanied similar forces in countries such as Turkey, Iran, Egypt and even Israel. That suggests it is these forces, foremost among them population growth and rapid urbanisation, which are primary, and the religious consequences which are secondary. This in turn suggests that "blaming" religion as such for unwelcome changes and for violent acts misses the point, which is not the same as saying that clerical elites do not have a responsibility to oppose distortions of religion.

As the Islamist movement in Turkey grew, some sympathetic observers a few years ago likened it to the Methodist movement in Britain in its early days. The immigrants from the countryside coming into Turkey's big cities, and the children born to them there, wanted a religious and political presentation, and representation, which catered to them. The older parties tried, had some success, but usually lost any credit they gained on the campaign trail in government.

Equally, perhaps, Turkish Islam, characterised by state control of the formal structures on the one hand and a freemason-like network of religious orders for the more privileged on the other, did not meet the needs of the new classes in the cities. It may be that the most important aspect of the recent triumph of the Justice and Development party is that it registers fully this demographic and social shift in Turkey rather than that it brings Islam into Turkish politics in a way not permitted before.

In Iran very similar social changes are having an almost opposite effect, as many have noted. The movement into the cities and the tensions between old and new kinds of businessmen contributed toward the revolution which removed the Shah. But population growth and urbanisation have since gone at a pace beyond anything seen in the Shah's time, so Ayatollah Khomeini's revolution has been faced with new classes and masses and their needs just as was the Turkish establishment. Since political Islamists are in power in Iran, frustration and anger with the government has taken on an inevitably secular character. A principled opposition among those clerics who were always doubtful about the legitimacy of Khomeini's innovations has also grown.

President Mohammad Khatami has kept political Islam going by promising reform, but the long struggle between his supporters and the hardliners may now be heading for some kind of resolution. A victory for the reformists might be a kind of Iranian equivalent to the Turkish changes, or, if the Iranian hardliners crack down, to the clash between the new government and the armed forces which is still a remote possibility in Turkey.

In Israel and the occupied territories, the same population growth and urbanisation have taken peculiar forms but are still profoundly influential. Palestinian population growth and the lumpen urbanisation represented by concentration in refugee shanty towns, on the one hand, and by commuting to work in Israel on the other, happened in areas both inside and outside Israel's control. Whichever it was, Israel faced problems it had not envisaged.

The Palestinian community in Israel, in the occupied territories and in the countries where Palestinians lived in exile, grew immensely in size, and maintained and strengthened its identity - exactly the opposite of what the Israelis had hoped. In the territories, the new classes had new demands like their equivalents elsewhere, although of course shaped by the distorted and impoverished conditions imposed by the conflict, and movements like Hamas and Islamic Jihad can be seen as responding to those demands in ways the Palestine Liberation Organisation and then the Palestine Authority could not or would not.

Meanwhile, Israel's own Jewish population growth and greater urbanisation, reinforced by immigration, played a part in the decline of the Labour party, better fortunes for Likud and the increased influence of the religious parties that have characterised its recent political history.

Egyptian developments show yet another possibility. Where there is little formal political access for those with apparently religious answers to political problems, and where the violent takeover of the state has both failed and been repudiated by a majority in the movement, new leaders responding to new needs can proceed by taking over the informal and non-governmental institutions of society. The success of Islamists in the Middle East can be accounted for by the inadequacy of existing governments, and the US blamed at one remove for sustaining such governments.

There is obviously some truth in this. But a fairer picture would be of politicians of many different stripes attempting to cope with, or capitalise on, large and destabilising changes, sometimes in honest and sometimes in dishonourable ways. The case of Iran shows that where Islamists have achieved power they are as subject to the vagaries of this prevailing political weather as are secular regimes. In the worst situations, both state violence and the freelance violence of rebels, revolutionaries and mobs spring out of these exigencies, although they obviously draw on the intolerant side of religious traditions. Those truly devoted to their religion must continue to try to moderate these effects. There is no denying that, whatever the ultimate causes, killing in the name of religion of the sort recently seen represents a moral breakdown.