Answer:
Mecca (مكة المكرمة) and Medina (المدينة المنورة)
Explanation ( Complete Oversimplification ):
The areas which are considered the centers of Islam are in Arabia in the cities of Mecca (مكة المكرمة) and Medina (المدينة المنورة). When the Ottoman Empire controlled these lands however, There was also the city of Konstantiniyyee / Constantinople (القسطنطينيه) which was the gateway from the Islamic world to the Christian world. Adding a little side note, Islam, like Christianity and Judaism, Is a RELIGION. It has had various locations over times from the Islands of Indonesia to Coasts of Morocco.
