A striking fact has changed my view of Islam: that for 400 years, between the Islamic victories of the seventh century until the end of the 11th century, a half of the world's professing Christians lived under Muslim rule.

Those were the centuries of St Bede and St Dunstan in England, of the foundations of the congregation of Cluny and the order of Chartreuse in France. The life of the other half of the body of Christians is quite unknown to those of us who do not read Arabic or Syriac. At best we have heard of St John of Damascus (died 749), who worked as a top civil servant in the Umayyad caliphate.

In place of knowledge a series of myths has grown. One is of a cultural golden age in Islamic Spain, with Muslims, Christians and Jews living in harmony. Yet Americo Castro, who coined the word convivencia to describe the life of the three faiths in the caliphate of Cordoba, wrote, in his book The Structure of Spanish History: "Each of the three peoples of the peninsula saw itself forced to live for eight centuries together with the other two at the same time as it passionately desired their extermination."

The tension was resolved in the reconquest of Spain by Christian forces and the later terrible expulsion of Jews and Muslims. Spain was the only territory long under an Islamic rule where Christians did not (from the pressure of taxation, law and periodic massacre) eventually dwindle into a helpless minority.

To be sure, philosophy and religious discussion flourished for a time in Baghdad under the Abbasid caliphate, established in ad 750. How this struck more hardline Muslims is shown in an account by a visitor from Spain, Abu Umar Ahamad ibn Muhammad ibn Sadi. "I witnessed a meeting which included every kind of of group: Sunni Muslims and heretics, and all kinds of infidels: Majus, materialists, atheists, Jews and Christians. Each group had a leader who would speak on its doctrine and debate about it," he recorded with a shudder. "I never went back." This snippet comes in a fascinating survey I mentioned two weeks ago, Sidney Griffith's The Church in the Shadow of the Mosque (Princeton, 2008).

He gives a vivid picture of how Christians contributed to intellectual life under the Abbasids, and how they developed their theology in response to the stimulus of Islam. Theirs was a rich and unsuspected world.

It is well known that the West rediscovered ancient philosophy, notably Aristotle, via Arabic translations. I had not realised that the majority of the translators of Greek texts into Arabic in the lively early Abbasid translation movement were Christians.

One translator was Patriarch Timothy I of the Church of the East. He lived in the generation after John of Damascus, and he transferred his see from ancient Ctesiphon in Persia to the new centre, Baghdad.

Patriarch Timothy, who ruled his church for 43 years, translated Aristotle's Topics for the caliph al-Mahdi, in whose court he conversed with other Aristotelian philosophers on knowledge and the doctrine of God.

Such discussions were not to endure. A straw in the wind was the anti-Christian policy of al-Mutawakkil (who reigned as caliph from 847 to 861). It was he who squashed the philosophically rich speculation within Islam exercised by the Mutazilite movement. In our own day, praise of the great Islamic philosophers Ibn Sina (Avicenna), al-Farabi or Ibn Rushd (Averroes) will win you few friends in orthodox Islamic circles.

The numbers of Christians in the territories of Islam declined with the 13th‑century Mongol invasions, followed by a rigorously hostile interpretation of proper Islamic relations with Christians, advocated by the permanently influential Ibn Taymiyya (1263-1328).

Christians who still speak Syriac, Arabic and Coptic, who have survived the centuries, deserve our understanding and help.

By Christopher Howse
www.telegraph.co.uk