William Dalrymple: Two civilisations entwined in history

'One of the most powerful Ottoman viziers was the eunuch Hasan Aga, formerly known as Samson Rowlie from Great Yarmouth'

Last weekend, before the bombing began, while the papers across Europe were still debating the resurgent argument about the clash of civilisations, I was wandering around Istanbul.

Passing along the Golden Horn in bright autumnal sunlight, I came across a magnificent tomb complex. A shady garden gave on to the courtyard of a mosque, behind which stood an octagonal tower, the mausoleum for the Ottoman admiral Kilic Ali Pasha. The Pasha, it turned out, had fought the combined navies of the West at the Battle of Lepanto in 1571 and was one of the few Ottoman commanders to distinguish himself. After this he was made Kaptan Pasha, or Lord High Admiral, and two years later helped to seize Cyprus from the Venetians.

Here, it seemed, was a figure who might be taken to epitomise Berlusconi's clash of civilisations – "The Terrible Turk" incarnate – until I read that Kilic Ali was, in fact, an Italian from Calabria called Ochiali who had converted to Islam. The easy assumption of some essential conflict between two very different civilisations became a little more nuanced still when I read that the complex had been constructed by another Christian convert – the great architect Sinan – and that the mosque he built was an almost perfect miniature of the Byzantine Cathedral of Holy Wisdom, Hagia Sophia.

Nor was this a unique case. At the same time as Kilic Ali was the Ottoman High Admiral, one of the most powerful Ottoman viziers was the eunuch Hasan Aga, formerly known as Samson Rowlie from Great Yarmouth. At the same time in Algeria, the "Moorish King's Executioner" turned out to be a former butcher from Exeter called "Absalom" (Abd-es-Salaam). And the Ottoman general known as "Ingliz Mustapha" was a Scottish Campbell who had embraced Islam and joined the Janissaries.

The links that bind Christianity and Islam are so deep and complex that the occasional confrontations should perhaps more properly be looked upon as a civil war between two different streams of the same tradition than any essential clash of civilisations. When the early Byzantines were first confronted by the Prophet's armies, they assumed that Islam was merely a variant form of Christianity, and in some ways they were not so far wrong; Islam, of course, accepts much of the Old and New Testaments, obeys the Mosaic laws about circumcision and ablutions and venerates both Jesus and the ancient Jewish prophets.

Indeed, the greatest theologian of the early church, St John of Damascus (d. 749), was convinced that Islam was at root not a new religion, but a variation on a Judaeo-Christian form. This perception is particularly remarkable as St John had grown up as a young Arab aristocrat in the Ummayad Arab court of Damascus – the hub of the young Islamic world – where his Orthodox Christian father was the Chancellor. St John himself was an intimate boyhood friend of the future Caliph al-Yazid, and the two boys' drinking bouts in the streets of Damascus were the subject of much gossip in the capital.

Later, in his old age, St John took the habit at the desert monastery of Mar Saba where he began work on his great masterpiece, The Fount of Knowledge. The book contains an extremely precise critique of Islam, the first ever written by a Christian, which, intriguingly, John regarded as a form of Christianity and closely related to the heterodox Christian doctrine of Arianism. (After all, this doctrine, like Islam, took as its starting point a similar position – that God could not become fully human without somehow compromising his divinity.)

This was a kinship that both the Muslims and the Christians were aware of. In 649 the Nestorian Christian Patriarch wrote: "These Arabs fight not against our Christian religion; nay, rather they defend our faith, they revere our priests and saints, and they make gifts to our churches and monasteries." This tradition continued and led to many surprising anomalies; Saladin's private secretary and the head of his war office were both Coptic Christians, as were the Egyptian commanders who defeated the Seventh Crusade in 1250.

Throughout the Middle Ages there were few, if any, conversions by the Sword, a myth much propagated in anti-Islamic literature and recently expounded on at length by Paul Johnson in an incredibly ignorant article in that flag-waver of the new bigoted Islamaphobia, The Spectator.

The longer you spend in the Christian communities of the Middle East, the more you become aware of the extent to which eastern Christian practice formed the template for the basic conventions of Islam: the Muslim form of prayer with its bowings and prostrations appears to derive from the older Syrian Orthodox tradition still practised in pewless churches across the Levant; the architecture of the earliest minarets, square rather than round, derive from the church towers of Byzantine Syria; and Ramadan, at first sight one of the most distinctive Islamic practices, is nothing more than an Islamicisation of Lent, which in eastern Christian churches still involves a gruelling all-day fast.

Certainly, if a monk from sixth-century Byzantium were to come back today, he would find much more that was familiar in the practices and beliefs of a modern Muslim Sufi than in, say, a contemporary American evangelical. Yet this simple truth has been lost by our tendency to think of Christianity as thoroughly Western, rather than the Oriental faith it actually is. We also forget that Islam inherited the same Greek and Roman foundations as our own culture; indeed the Muslims preserved the classics for us, before passing them back via the universities of Islamic Spain and Sicily.

The recent tendency to demonise Islam – and we have seen a great deal of that recently, especially from The Telegraph and The Times – has led to an atmosphere where few in either camp are aware of, or indeed wish to be aware of, the kinship of Christianity and Islam. Yet this essential kinship is something Muslim writers have been aware of for centuries. Jalal-ud Din Rumi, the 13th-century Sufi (d.1273), was perhaps the greatest of all the mystical writers of Islam, and lived in Konya, in Anatolia, at a time when its population was almost equally divided between Muslims, Christians and Jews.

When he was asked about the relationship between these three apparently incompatible religions, he told a story about a city of the blind:

One day the news came that an elephant was passing outside the city, so the townsfolk decided to send a delegation to report back as to what an elephant was. Three men left and stumbled forwards until they found the beast. They felt the animal and headed back to report. The first man said: "An elephant is like a vast snake!" The second man was indignant at hearing this: "What nonsense!" he said. "I felt the elephant and what it most resembles is a huge pillar." The third man shook his head and said: "Both these men are liars! I felt the elephant and it resembles a broad, flat fan." All three men stuck by their stories and for the rest of their lives refused to speak to each other. Each professed that they and only they knew the truth.

Of course all three blind men had a measure of insight. The first felt the trunk of the elephant, the second the leg, the third the ear, but not one had begun to grasp the totality or the greatness of the beast. If only they had listened to one another, they might have grasped the true nature of the beast. But they were too proud and preferred to keep to their own half-truths.

"So it is with us," said Jalal-ud Din. "We see the Almighty one way, the Jews have a slightly different conception and the Christians a third. To us, all our different visions are irreconcilable. But what we forget is that before God we are like blind men stumbling around in total darkness..."

The writer's most recent book is 'From the Holy Mountain: travels in the shadow of Byzantium' (Flamingo, £8.99)