Main article: Religion in Istanbul
Further information: Mosques, Churches, Synagogues
The urban landscape of Istanbul is shaped by many communities. Among all major religions that are practiced in the city, the one with the most populous community is Islam. The first mosque in Istanbul was built in Kadıköy (ancient Chalcedon) on the Asian side, in the mid 14th century; a full century before the conquest of Constantinople across the Bosporus, on the European side, by the Ottoman Turks in 1453. The first mosque on the European side of Istanbul was built inside the Rumeli Castle in 1452. The first grand mosque which was built in the city proper was the Eyüp Sultan Mosque in around 1459. The mosque was built on the site of the grave of Abu Ayyub al-Ansari, a companion of the Prophet Muhammad who had died fighting in the Arab army in a siege to take the city in 669, making Istanbul one of the holy cities of Islam. The first imperial mosque inside the city walls was the Fatih Mosque (1470) which was built on the site of the Church of the Holy Apostles, an important Byzantine church which was originally edificed in the time of Constantine the Great. Many other imperial mosques were built in the following centuries, such as the famous Süleymaniye Mosque (1557) which was ordered by Suleiman the Magnificent and designed by the great Ottoman architect Sinan, and the famous Sultan Ahmet Mosque (1616) which is also known as the Blue Mosque for the blue tiles which adorn its interior.
Istanbul was the final seat of the Islamic Caliphate, between 1517 and 1924. The personal belongings of Mohammed and the earliest Caliphs who followed him are today preserved in the Topkapı Palace, the Eyüp Sultan Mosque and in several other prominent mosques of Istanbul. The seat of the Patriarch of Constantinople, spiritual leader of the Greek Orthodox Church is located in the Fener (Greek: Φανάρι) district. Also based in Istanbul are the archbishop of the Turkish-Orthodox community, an Armenian archbishop, and the Turkish Grand-Rabbi.
According to the 2000 census, there were 2691 active mosques, 123 active churches and 26 active synagogues in Istanbul; as well as 109 Muslim cemeteries and 57 non-Muslim cemeteries. Religious minorities include Greek Orthodox Christians, Armenian Christians, Catholic Levantines and Sephardic Jews. Some neighbourhoods have sizeable populations of these ethnic groups, such as the Kumkapı neighbourhood, which has a sizeable Armenian population, the Balat neighbourhood, which has a sizeable Jewish population, the Fener neighbourhood which has a sizeable Greek population, and some neighbourhoods in Nişantaşı and Beyoğlu, which have sizeable Levantine populations. In some quarters, such as Kuzguncuk, an Armenian church sits next to a synagogue, and on the other side of the road a Greek Orthodox church is found beside a mosque.