Istanbul has been the seat of the Ecumenical Patriarchate since 4th century AD. It does continue to serve as the seat of some other Orthodox churches, such as the Turkish Orthodox Church and the Armenian Patriarchate.
After moving the capital of Roman Empire to Constantinople, this city have become the capital of Christianity nearly for 1000 years. So Byzantines had built many imperial churches like Hagia Sophia and others. So Istanbul have become like home of Eastern Orthodox Christianity.
After the Turkish conquest of Constantinople in 1453, the conqueror Mehmed II founded the Millet System. This Millet System had governed these ethnic groups during Ottoman Empire based on faith. For this purpose, Sultan Mehmed II also established previously non-existent religious authorities. Armenian Patriarchate of Constantinople was one of them from 1461. A great deal of the internal affairs regarding these communities had been assigned to the administration of those religious authorities: such as Ecumenical Patriarchate, Armenian Patriarchate and later the Chief Rabbi for the affairs of Jews.
Christians in Istanbul
Today, most of Turkey’s remaining Greek and Armenian minorities live in or near Istanbul.
Beside, the mostly Catholic Levantines ( Catholic Christianity) traders founded trading outposts during the Byzantine and Ottoman periods. They are the descendants of European (Genoese, Venetian and French). A number of places reflect past movements of different communities into Istanbul, like Arnavutkoy, Polonezkoy and Yenibosna.
The Wealth Tax of the World War II years imposed higher tariffs on non-Muslims and foreigners in Turkey and played an important role in the migration of Istanbul’s Italians to Italy. The influence of the Italian Community of Istanbul is still visible in the architecture of many quarters. This architecture is particularly in Galata, Beyoglu and Nisantasi. (Christianity)