Paganism flourished in Istanbul before Christianty was established in the 4th century, when churches began to appear. The first churches in Istanbul were the Havariyun Church, Hagia Sophia and Hagia Irini. Many churches were built up to the time of the Ottoman Conquest and many sects were represented, i.e. Nestorians, Monophysites, Catholics, Orthodox, Assyrian, Gregorian, Dominican and Francisians. Also the Greek , Armenian, Latin and Genoese built their own churches.

After the conquest Sultan Mehmed, the Conqueror left the churches free to worship, but m the Ottoman Era many church buildings were sold and those which were derelict or abandoned were purchased and converted to mosques. The construction of churches nevertheless continued during the Ottoman Period but most were built outside the city walls in places such as Beyoğlu and summer resorts as well as along the shores of the Bosphorus. After the 19th centrury most new churches were located close to the embassies. The prominent ones are Saint Maria Draperis, St. Antoine de Padoue, Latin Italian Church, Armenian-Catholic Surp Yerrontutyan Church, Terre-Samte Spanish and the British Embassy Church.

Similarly synagogues in Istanbul are as old as the churches. The first known synagogue was built in 318 A.D.. Synagogues managed to survive in spite of the fact that some were converted to churches from time to time or were pillaged during the Latin Occupation in the thirteenth century. The number of synagogues in Istanbul increased during the Ottoman period. With the migration to Istanbul of Jews from Spain and other parts of Europe as the result of the Inquisition m the 15th century A.D., the number of synagogues increased even further.

Synagogues built during the Ottoman period did not have a distinctive architectural style. They all share a very non-decorative, simple appearance and were built in courtyards in a plain rectangular shape. Many synagogues constructed during the Ottoman period are still active and serving the Jewish community in Istanbul today.