When Pope Benedict visited Istanbul yesterday, he entered what is now called the Aya Sofya museum in Istanbul. Its transition from church to mosque to secular hall testifies to the central role religion has played in this ancient city.
Here are some key facts on Istanbul:
• Where is it? - Istanbul is split in two by the Bosphorus Straits linking the Black Sea to the Sea of Marmara and then to the Aegean and Mediterranean Seas. Each side of the city lies in a different continent, the western half in Europe, the east in Asia.
• Population: 10 million.
• Changing times, changing names: - Greek colonists led by King Byzas founded the city around 600 BC, naming it Byzantium.
• Emperor Constantine, who embraced Christianity, made the site the capital of the Roman Empire in 330 AD and it became known as Constantinople.
• Christianity split in the Great Schism of 1054, when the eastern Orthodox patriarchs refused to recognise papal authority and the Pope in Rome ex-communicated them.
• Western crusaders sacked Constantinople in 1204, cementing the split between Catholic west and Orthodox eastern churches. In 2004, 800 years later, the Vatican returned the looted remains of two saints as a gesture of goodwill.
• In 1453, Sultan Mehmet II captured Constantinople and made it the capital of the Ottoman Empire, an Islamic state that ruled much of the Middle East, North Africa and southeast Europe for 450 years. Ottoman sultans later claimed the title of caliph, the spiritual and temporal ruler of the Muslim world as successors of the Prophet Mohammad.
• People from all over the empire moved to the Ottoman capital and Muslims, Jews and Christians made up a cosmopolitan society.
• The Ottoman Empire welcomed Jews expelled from Spain in 1492 and many settled in the capital.
• Mustafa Kemal Ataturk founded the Turkish Republic in 1923 after the defeat of the Ottoman Empire in World War I. He dissolved the Caliphate and established a secular state based in Ankara and officially renamed the former capital Istanbul.
• Religion: - Some 99 per cent of Istanbul citizens are either Sunni or Alevi Muslims.
• The city is still the seat of the Ecumenical Patriarch, recognised as first among equals by most of the worldwide Orthodox Church. But Turkey's Orthodox community dwindled from about 1.5 million before a 1923 population exchange with Greece, to 200,000 before emigration and expulsions in the 1950s and 1960s, to just around 3,000 now.
• There are sizeable Jewish and Armenian minorities in Istanbul numbering in the tens of thousands.