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 .
There are at least five different religious groups in Istanbul .