The decline of Christianity in the Middle East did not occur over night. It took over a millennium for Christians to go from being a slight majority of the population of the Middle East to being 5 percent at present.
When did it start? We see the beginnings of decline by the 9th century. Initially, the Muslim conquerors discouraged the People of the Book, Christians, from converting to Islam. The benefits which the conquerors enjoyed were initially not given to non-Arab converts to Islam. But as the caliphate developed into a multi-ethnic, multi-religious state, those privileges were granted to all converts, providing incentives for Christians to convert to Islam.
Under the Abbasid Caliphs (750-1258) there were often dialogues/debates in which Christian and Muslim theologians debated the superiority of their own faiths. By the 9th century, though, we notice subtle and not so subtle changes taking place. Since many of these debates have survived in written form, we can follow their development. (You can read more about that here.) Although for the most part, there was no violence exerted on Christians to convert to Islam, subtler and perhaps more powerful incentives were at work: social status, political status and financial advantage. While there does not seem to have been any mass conversion to Islam, the decline had begun.
At the beginning of the 13th century, nomadic groups in north central Asia began a migration and conquest. In 1258, Mongols conquered Baghdad and brought the Abbasid Caliphate to an end. By 1300, they had conquered China, all of Central and Western Asia and parts of Eastern Europe. Until the advent of ISIS, the Middle East had not experienced wanton destruction such a scale since the Mongol invasions. The infrastructure of the multi-ethnic, multi-religious caliphate was destroyed — and with it, most of the Christian infrastructure in the region.
To be sure, there would not be a permanent vacuum in the Middle East. Smaller groups of Muslims formed Khanates and Sultanates which often competed with each other for the position which the Abbasid Caliphate had possessed and which was now gone forever.
Even towards the end of the caliphate, one notices the presence of Turkic people in positions of power. After the demise of the caliphate, more and more of these groups move into the Middle East and begin to establish governments.
One of these groups, the Oghuz Turks, was to begin the most important empire in Europe and Asia for almost 600 years. The Osmanli family formed a dynasty, the Ottomans, which would conquer the entire Middle East and much of Central Europe until its dissolution in 1923. The Ottoman Empire was inspired by the ghazi or raiding tradition and engaged in almost constant raids against their non-Muslim and Muslim neighbors.
In many ways, the situation of Christians and non-Ottoman Muslims under the Ottomans was similar. Both were subjects. However, there were several factors which made the situation of Christians worse. Christians were becoming a minority lacking the “demographic depth” of the Arab Muslims, making any recovery very difficult. The Crusades (roughly 11th to 13th centuries) and the later constant Ottoman attacks in Christian central Europe gave Christians in the Ottoman Empire the air of being “the foreign enemy” or, worse, a “fifth column.” Forced conversions of Christians to Islam had been rare. However, the Janissaries, the Ottoman shock troops, were originally manned by young Christian boys kidnapped from (mostly) Balkan countries, forced to convert to Islam and then trained as the personal troops of the Sultan.
Relations between Christians and Ottoman Turks are memorialized in several Catholic holy days. The Feast of the Name of Mary (12 September) commemorates the defeat of the Ottoman armies at Vienna in 1683 and the Feast of the Holy Rosary (7 October) commemorates the Christian victory at the naval battle of Lepanto in 1571. It’s no accident that these holy days are linked to battles; the Ottoman Empire was in an almost constant state of war with Eastern Europe until the end of the 17th century.
The time of the Umayyad and Abbasid Christian-Muslim dialogues was over. For the Christians in the West, Islam — in the form of the Ottoman Empire — was not an opportunity for dialogue, but a threat. Christians in the East were considered objects of suspicion — sympathizers of the Christian powers in Europe — and not people for dialogue.
Christians in Syria and Mesopotamia never really recovered from the Mongol invasions and destruction. Any opportunity to recover was greatly diminished under the Ottomans. Christians went from being high officials and respected scholars under the caliphates to being a poor, shrinking and suspect minority under the Ottomans.
As we will see, the events of the 20th and 21st centuries would make the situation of Christians in the Middle East even worse.
Related:
2,000 Years of Christianity in Syria and Mesopotamia: Introduction
2,000 Years of Christianity in Syria and Mesopotamia — Part 1: In the Beginning
2,000 Years of Christianity in Syria and Mesopotamia — Part 2: Christians and Muslims Co-exist