Although small Armenian, Greek Orthodox, Jewish, and Catholic communities survive, the majority of Istanbullus are Muslim, and adhere to the principles known as the  Five Pillars of Islam   to believe with all one's heart that  There is no God but God, and Mohammed is his Prophet ; to pray five times a day, at dawn, midday, afternoon, sunset, and after dark; to give alms to the poor, and towards the upkeep of the mosques; to fast between sunrise and sunset during the month of Ramadan; and to try to make the pilgrimage to Mecca at least once in one's lifetime.
The majority of Istanbullus practice Catholicism.