Today the West often views Islam as a civilisation very different from and indeed innately hostile to Christianity. Only when you travel in Christianity’s Eastern homelands do you realise how closely the two are really connected, the former growing directly out of the latter and still, to this day, embodying many aspects and practices of the early Christian world now lost in Christianity’s modern Western-based incarnation. When the early Byzantines were first confronted by the Prophet’s armies, they assumed that Islam was merely an heretical form of Christianity, and in many ways they were not so far wrong: Islam accepts much of the Old and New Testaments and venerates both Jesus and the ancient Jewish prophets.
Significantly, the greatest and most subtle theologian of the early church, St. John Damascene, was convinced that Islam was at root not a separate religion, but instead a form of Christianity. St. John had grown up in the Ummayad Arab court of Damascus, where his father was chancellor, and he was an intimate boyhood friend of the future Caliph al-Yazid; the two boys’ drinking bouts in the streets of Damascus were the subject of much horrified gossip in the streets of the new Islamic capital. Later, in his old age, John took the habit at the desert monastery of Mar Saba where he began work on his great masterpiece, a refutation of heresies entitled the Fount of Knowledge. The book contains an extremely precise and detailed critique of Islam, the first ever written by a Christian, which, intriguingly, John regarded as a form of Christian heresy related to Arianism: after all Arianism, like Islam, denied the divinity of Christ. Although he lived at the very hub of the early Islamic world, it never seems to have occurred to him that Islam might be a separate religion. If a theologian of the stature of John Damascene was able to regard Islam as a new- if heretical- form of Christianity, it helps to explain how Islam was able to convert so much of the Middle Eastern population in so short a time, even though Christianity remained the majority religion until the time of the Crusades.
— Taken from The Holy Mountain: A Journey Among the Christians of the Middle East, by William Dalrymple