Throughout history Istanbul has been a home for people with different backgrounds. As a result, you can find neighborhoods housing people with different religions. According to 2000 census, there are 123 active churches, 20 active synagogues, and 2691 mosques in Istanbul. The difference in numbers is due to the fact that non-muslim people only constitutes around 2% of the entire population in Turkey. They are free to practice their religion, and this is stated in the secular constitution of Republic of Turkey.
Photo of Pammakaristos Church in Istanbul.