Istanbul has been the seat of the Ecumenical Patriarchate since 4th century AD, and does continue to serve as the seat of some other Orthodox churches, such as the Turkish Orthodox Church and the Armenian Patriarchate.
After the Turkish conquest of Constantinople in 1453, the conqueror Mehmed II founded the Millet System. According to this system, different ethnic groups within Constantinople and the rest of the country had been to be governed by a group of institutions based on faith. For this purpose, Sultan Mehmed II also established previously non-existent religious authorities such as the Armenian Patriarchate of Constantinople in 1461. With the Millet System, a great deal of the internal affairs regarding these communities had been assigned to the administration of those religious authorities: such as Ecumenical Patriarchate for the affairs of all Orthodox Christians, Armenian Patriarchate for the affairs of Armenians and later the Chief Rabbi for the affairs of Jews.
Today, most of Turkey’s remaining Greek and Armenian minorities live in or near Istanbul.
Beside, the mostly Catholic Levantines who are the descendants of European (Genoese, Venetian and French) traders founded trading outposts during the Byzantine and Ottoman periods. 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, particularly Galata, Beyoglu and Nisantasi.