The telephone call came just as I’d finished presenting the Today programme. If I were invited to lunch with a certain person, would I undertake to treat our conversation as confidential? Probably not, I said, but it depended on who the person was.

It turned out to be the head of the secret intelligence service, MI6. I went to lunch. That was 13 years ago — in the days before spy chiefs published their memoirs and when hacks like me weren’t even supposed to know who the top spook was.

There was only one question for him over lunch. The cold war was won and the Warsaw Pact had collapsed, so who would be the new enemy and where would the threat come from? The threat, he said, was cyber-terrorism. The enemy would be militant Islam. Not bad: one wrong (so far) but one horribly right.

David Blunkett did not need to spell it out in the House of Commons last week. He would not say exactly why the tanks and troops had been sent to Heathrow or where the threat had come from, but we all knew. We all had the same image in our minds. It has been there since September 11. It is only Muslim terrorists, we believe, who have the wish to shoot down a jumbo jet as it climbs into the sky over Windsor.

Perhaps my spook was not being terribly clever. He need only have read a little history to have reached his conclusion. Over the past 1,000 years or so, when Christian Europe has not been fighting itself, it has been at war with Islam. That’s what the crusades were about. That’s why the Moors invaded Spain — and were thrown out again. Perhaps the old struggle has merely changed its form.

In today’s climate of fear perspectives are jolted. What we thought we understood from the little pictures gets challenged by our failure to understand the big one. Our friendship with the nice Muslims down the road or the ever-helpful newsagent on the corner has to be re-examined in terms of the emerging monolith called Islam.

This is how demonising happens. The Germans who lived happily in Britain for so many years and loathed Hitler just as much as we did became objects of suspicion in 1939. The Japanese who had settled so well on the West Coast of America were locked up in internment camps after Pearl Harbor. We need to be on our guard against demonising as much for our own sake as anything else.

The danger is seeing a monolith where none exists. Islam is no more monolithic than Christianity. Dr Rowan Williams and Ian Paisley worship the same God — and that’s where any similarity begins and ends. Or try comparing your local laid-back Anglican church with the Bible-bashing fundamentalists of the American Deep South.

Islam has its own divisions.Throughout the 19th century, as the Ottoman empire withered, it was clear that Islam had lost out to the richer and more powerful Christian West. Abdal Hakim Murad, an Islamic scholar at Cambridge, says most Muslim leaders reacted to that by trying to find ways of reconciling Islam with the new situation. They were the “accommodationists” and they were the predominant force within Islam — until oil began to speak.

The first oil crisis of 30 years ago gave enormous power and even more wealth to those Arab nations who were sitting on top of the world’s greatest reserves. They happened to be the countries whose religious leaders had largely rejected the accommodationist approach. Capricious fate indeed.

The Wahhabi Muslims of Saudi Arabia have been compared to 16th-century Calvinists in Christianity: scriptural literalists who wanted to rely solely on the sacred texts where everything you need to know can be found without the presumptuous mediation of theological interpreters. In Britain, at least, Calvinism eventually disappeared to the margins. In Islam it turned out differently. Osama Bin Laden is an Islamic Calvinist.

Islam has ended up with two other groups besides the radical literalists: the traditionalists, who regard the fundamentalists’ theological views as heresy, and the heirs of the accommodationist reformers, who are horrified at how the fundamentalists interpret the Koran to justify violence against non-Muslims. There are many more reformers than fundamentalists.

It’s worth remembering that after September 11 every significant Muslim leader around the world condemned the attack — whatever Margaret Thatcher may have said at the time about a failure to do so. In Tehran’s football stadium they observed a minute’s silence. Malaysia is only one country that has every intention of being both Islamic and part of the current global order.