Do you know your Sunnis from your Shias? The distinction between these two varieties of Muslims has become increasingly important. So, I hope you are not among the honourable members of the Mother of all Parliaments who think al-Qaeda is Shia.
A spot check by the Sunday Times exposed the extent of ignorance among our MPs. Many of the 30 MPs it polled, said to be "experts in the world of politics", and involved in various ways with the Middle East and Britain's Muslim community, could not tell the difference between Hamas and Hezbollah. But they need not worry too much. Neither can most people - including Muslims.
Indeed, the Sunday Times got into a great tangle. In its own answer to the question, the paper noted that the Shia/Sunni distinction "goes back to a dispute over who had spiritual authority over the Muslim community after the death of the Prophet Muhammad". Not quite. The dispute was just as much about political as spiritual authority - that is to say, who should become caliph and provide both spiritual and political leadership. "Shias believe," the paper correctly pointed out, "that authority was vested in his family." But the Sunday Times is totally wrong to say that only "Sunnis look to the sayings and actions of Muhammad as well as the Koran". For Shias as well, the Koran and the Prophet Muhammad are the epitome of authority - bar none.
Distinguishing between Sunnis and Shias is a bit like trying to tell the difference between butter and margarine. Both can be used to lubricate bread and they can taste equally good, so that it is not easy to tell which is which. All Muslims, Shia and Sunni, share exactly the same beliefs. Their religious practices, with a few minor variations, are also the same: they go to the mosque to pray, fast in the month of Ramadan, offer the obligatory religious charity, and go to Mecca for pilgrimage. The differences are quite irrelevant for the vast majority of Muslims. Indeed, during most of my life, I have never bothered to find out whether any one of my Muslim friends is a Sunni or a Shia.
But the geopolitics of the contemporary Middle East has changed all that. The intrinsic difference between butter and margarine has come to the fore: while one is natural, the other is an artificial creation. The problem of telling the difference, however, still remains. The battle over butter and margarine in Islam is not about theological difference, but about who will dominate the region: Saudi Arabia or Iran. Each of these two centres of Islam claims to be butter - the natural offspring of the Prophet's legacy and its true guardian - and describes the other as a mock, inferior product, hazardous to Islamic health.
As the real differences are so insignificant and marginal, each side has to describe the other in extreme, demonic terms in order to have any chance of influencing the minds of ordinary Muslims. Hence last month's fatwa by the high-ranking Saudi cleric Abdul Rahman al-Barak. "By and large," he declared, "Shias are the most evil sect of the nation and they have all the ingredients of the infidels." They should be treated, he suggested, as "apostates and hypocrites". Al-Barak's fatwa is only the latest in a long line of such denunciations from Saudi Arabia.
The Shia scholars in Tehran and Qom have responded in kind. They describe Saudi Arabia as an infidel agent of the infidel United States. The Saudis, Iran has repeatedly suggested, are unfit to rule Mecca and Medina, and the holy cities should be taken from them. They have threatened largely Sunni Pakistan with a Shia revolution, and gave their blessings to the Syrians when they were massacring the Sunni Muslim Brotherhood in Hama - a massacre that I witnessed in 1982. Ayatollah Khomeini, founder of the Shia theocracy, had little time for the Sunnis. Iran's obnoxious Revolutionary Guards regard most Sunnis with suspicion and derision.
To such fanaticism, a rational Muslim - Sunni or Shia - could have only one response: a plague on both your houses. This is the position of most British Muslims, even if it is not fully articulated. The relationship between Shias and Sunnis in Britain has been harmonious. We could not care less about a 1,400-year-old dispute over who should have been caliph of the then emerging Muslim world, but we are horrified by the sectarian violence in the Middle East, and hear with trepidation the talk of "civil war" within Islam. Expect a joint statement from the Muslim groups making this clear in the coming weeks.
Ziauddin Sardar's "What Do Muslims Believe?" is published by Granta Books (£6.99)