From the founding of Constantinople in 324 AD to the middle of the eleventh century AD, Christianity was a dominant religion in the Middle East. Believed to be founded by the Apostles Peter and Paul, the Orthodox Church, located in Antioch (present day Turkey, close to the Syrian border), then a part of the Byzantine Empire, was the leading center of Christian learning.
Around 632 AD, there was rise of Islam in Arabia. In 634 AD, when the Arabs conquered Baghdad, and when they started attacking the Persian Empire in 633 AD, the Christian world watched gleefully. The Christians were seeing Islam as their ally against Zoroastrianism, which was the official religion of the Persian Empire. They cheered the fall of the Persian Empire in 654 AD.
When the Arabs conquered Jerusalem, the birthplace of Jesus, between 636 and 637 AD, the Christian world was shocked. Between 639 and 698 AD, the Arab armies swept across North Africa, conquering Egypt and Carthage. These defeats were a loss of prestige for the Orthodox Church and the biggest Christian empire of the region, the Byzantine Empire. The Christians realized that Islam had become a serious contender for power in the Middle East.
Until the second decade of the eighth century the Orthodox Church and the Byzantine Empire bore the brunt of the Islamic attacks—the Roman Catholic Church did not suffer any territorial losses, since it was headquartered in Europe. The Europeans had a taste of Islamic power from 711 AD, when the Arab armies crossed the Strait of Gibraltar and conquered the Iberian Peninsula (much of Spain and Portugal).
Before the rise of Islam, Orthodox Christianity had a philosophical and tolerant character. The Catholics used to accept the position taken by the Orthodox Church on most doctrinal matters. The relentless struggle with Islam transformed Orthodox and Catholic Christianity. By the tenth century, both schools had become fanatical and intolerant.
A race for gaining new adherents began in the Orthodox and Roman Catholic Churches. Orthodox Christianity expanded in Eastern Europe and Russia. The Roman Catholic Church expanded into Western Europe. One of the reasons behind the Catholic success in converting Western Europe was that the Catholic Pope had the support of the Holy Roman Emperor, Charlemagne. The Pope also enjoyed the financial backing of the wealthy traders of Venice, Genoa, Pisa, and Florence.
The Byzantine Empire, which was engaged in a war of survival against the Islamic armies, could not offer the same kind of support to the Orthodox Church.
Further damage was caused to the position of the Orthodox Church in 1054, when Pope Leo IX excommunicated the Patriarch of the Orthodox Church over a doctrinal issue (known as the Great Schism or Schism of 1054). By orchestrating the Patriarch’s excommunication, Leo IX had proved that the institution of the Roman Catholic Pope possessed the ultimate authority on all doctrinal issues—if the Patriarch of the Orthodox Church failed to accept the line taken by the Roman Catholic Pope then he could be excommunicated.
The Byzantine Empire was expected to support the Orthodox Church in their doctrinal dispute against the Roman Catholic Church. But the Byzantine Emperors were engaged in a struggle for survival against the Islamic movements. In 1071, the army led by the Byzantine Emperor Romanus IV Diogenes was defeated by the Seljuq Turks at the Battle of Manzikert. This defeat put an end to the notion that the Byzantine Empire was the greatest power in the Middle East. It was an empire in decline.
In 1094, the Byzantine Emperor Alexios I Komnenos appealed to the Roman Catholic Pope Urban II for assistance in the struggle against the Islamic forces. In response, Urban II launched the crusade to liberate the holy land and open new land routes to India.
The crusaders managed to liberate Jerusalem in 1099. But in 1144, the Turkish armies captured Edessa in Asia Minor, causing a breakup in the line of communication between Jerusalem and the European kingdoms. It was now logistically impossible to supply and defend Jerusalem. When the city fell to Sultan Saladin in 1187, the Christian world was outraged. A new crusade was launched to liberate Jerusalem and to open new land routes to India. This crusade too failed.
By the middle of the fifteenth century, the Middle East and North Africa were under Islamic control. With the loss of these regions, the Orthodox Church became less important. With its Orthodox rival in decline, the Roman Catholic Church became the leader of Christianity.