Religious sights of a city tend to connect you with the sacred side of local culture. Especially in Istanbul there is a delicate mosaic of a multi-cultural past. Istanbul and Anatolia in general has been home to many different civilizations and cultures for over centuries. As such, each civilization left a mark of its culture and religion on these lands.
Some of these marks (and monuments) were demolished during the succession of new rulers and new civilizations. Some of these did not perish, but evolved as in Hagia Sophia Mosque (which was once a church). And, some of these survived and they took their place next to a Mosque or a Muslim Palace. It is such that Istanbul has a unique, yet multi-cultural and mystifying religious carpet.
If you love history, do read a lot before you go to see the following landmarks.
Karantina Sok., Karaköy
Yeni Cami, Eminönü, 34116 Rüstempaşa, 34116, Turkey
At the corner of Çinili Mescit Sok. and Çavusdere Cad