Pope Francis visit: Turkey's Christians face tense times
It tells of a city where empires, cultures and religions collided. A building that bears mosaics of Jesus and the Virgin Mary beside calligraphy reading "Allah" and "the Prophet Mohamed". There is no greater symbol of the clash of civilisations here than Hagia Sophia.
For almost 1,000 years it stood as the most important Orthodox cathedral in the world, the religious heart of the largely Christian Byzantine empire whose capital was then called Constantinople.
But in 1453 the city fell to the Ottomans, Hagia Sophia became a mosque and Christianity began its slow demise here.
As Turkey grew out of the ashes of the Ottoman Empire, that decline accelerated. When Pope Francis arrives here this week, he will visit a country whose population has fallen from 20% Christian 100 years ago to around 0.2% today.
'Huge brain drain'
"No country in the region - including Iran - is as homogenous in terms of Islam as Turkey," says writer Cengiz Aktar. "It's a mono-colour country - it's a Muslim country."
After the Turkish Republic was born in 1923, it carried out a "population exchange" with Greece to create more ethnic and religious consistency. More than a million Greeks were forced out of Turkey to Greece while around 300,000 Muslims from Greece were relocated here.
The Greeks of Istanbul were initially saved but after a crippling wealth tax, anti-Greek pogroms in 1955 and mass expulsions in 1964, the Greek community was left in tatters. And so was the Orthodox Christianity they practised.
"The ethnic cleansing of these non-Muslim minorities was a huge brain drain," says Mr Aktar, who has created a new exhibition on the loss of the Greeks here.
"It also meant the disappearance of the bourgeoisie because not only were they wealthy but they were artisans. Istanbul lost its entire Christian and Jewish heritage."
It was not just the exodus of the Greeks that hit Christianity here.
Armenians were the other large Christian community. Hundreds of thousands were deported in 1915. They were either killed or died from starvation and disease. The label "genocide" is rejected by the Turkish state. From a population of two million Armenians, around 50,000 remain today.
Robert Koptas shows me around the office of his Armenian weekly newspaper, Agos. In 2007, the editor, Hrant Dink, was murdered outside by Turkish nationalists. Seven years on, Mr Koptas says the small Armenian community feels intimidated.
"Armenians fear expressing their religious identity here," he says.
"Most of the believers hide their cross inside their shirt. They can't open it and walk freely on the street because they could prompt a reaction. I don't want to say all the Turkish population is against Christianity but nationalism is so high that people are afraid to express themselves."
That is now the worry among the Christian minority here: that Turkish Muslim nationalism has grown under the Islamist-rooted government of Recep Tayyip Erdogan, prime minister for 11 years before being elected president last August.
Mr Erdogan has made moves to support the Christians, such as passing a law to return confiscated state property to them and allowing Christian religious classes in schools. But he constantly stresses his Islamic identity, his support base is conservative Muslim and he whips up the nationalists here, the mood hardening against Christians.
Catholics, the smallest Christian minority in Turkey, have felt the impact.
A spate of murders of Catholic missionaries and priests a few years ago left the community in shock. At the Catholic basilica in Istanbul, there is Mass for the few.
"To be a Turk now means you have to be Muslim," says Father Iulian Pista, who serves here.
"In the past, being a pious Muslim was looked down upon. Now Friday prayers are encouraged. Society here is becoming Islamised. Recently, I've seen youngsters defecate and urinate in my church. They shout 'Allahu akbar' [English: God is great]. I also believe God is great but the way they say it is threatening."
Islam was sidelined from the constitutionally secular Turkish republic founded in 1923. But as a nation state was formed here, the religion became part of Turkish national identity, something that has sharply accelerated under Mr Erdogan's leadership.
New mosques are flourishing, while the world-famous Halki Orthodox theological school near Istanbul has remained closed since 1971 under Turkish nationalist pressure. One of the remaining Greeks of Turkey, Fotis Benlisoy, says the community feels squeezed: "The threatening feeling for non-Muslim minorities here is coming again.
"There are many reasons: language and policies of the government, the president and prime minister using more conservative references to Sunni identity, pejorative words for non-Muslim communities coming from members of the cabinet, so much circulating about Turkey's relations with Isis [the Islamic State militant group based in Syria and Iraq] - all of this is making us think we might need an escape strategy."
At the magnificent Panaghia Greek Orthodox Church in Istanbul, the morning liturgy is led by Bartholomew I, "ecumenical patriarch of Constantinople", a position still based here.
It is a reminder of this country's heritage - and of a Christian faithful that is small but defiant. As modern Turkey builds its identity, the question still remains: can it embrace true religious freedom - or will nationalism stand in the way?