That's when Istanbul was called Constantinople and was the most important city in the Christian world.
Photos: The Patriarch
On The Road: Traveling Back in Time
Full Segment: The Patriarch
Web Extra: Cappadocia
Web Extra: Pilgrimage
Web Extra: The Footsteps of Christ
But times change, and in modern Muslim Turkey the patriarch doesn't feel very welcome. Turkish authorities have seized Christian properties and closed Christian churches, monasteries and schools. His parishioners are afraid that the authorities want to force Bartholomew and his church - the oldest of all Christian churches - out of Turkey.
His official title is impressive: "His All Holiness, Bartholomew, Archbishop of Constantinople, New Rome, Ecumenical Patriarch."
"Ecumenical" means "universal," and worldwide, 300 million Orthodox Christians look to him for spiritual guidance.
60 Minutes correspondent Bob Simon first met him in Istanbul. It was Easter, and worshipers from throughout the Orthodox Christian world had come to celebrate Christ's resurrection on the holiest day of their calendar with the man who they see as their pope.
"My first question is this. How should I refer to you? As your all holiness? As patriarch? As ecumenical patriarch? What is the proper way to address you?" Simon asked the patriarch.
"The official title is 'your all holiness,'" he replied, laughing. "But for me, Bartholomew is enough."
For him perhaps, but not for us. And while his all holiness may occupy the Ecumenical throne, his quarters are a far cry from the Vatican.
His office is cramped and relatively austere, his desk littered with papers. No Michelangelos there. All that is left of a Christian empire once ruled from Istanbul is a complex of nine buildings, tightly squeezed onto less than an acre of land.
"Now, is this the Vatican of your church?" Simon asked.
"Well, our headquarters," the patriarch replied.
It's called the Phanar, and it has been the heart of Orthodox Christianity since 1599.
His all holiness promotes an informal atmosphere there: there's none of the ritual that surrounds the pope in Rome. And there is no cathedral - only a modest church. The neighborhood that surrounds it used to be Greek and Christian but today is predominantly Muslim. The Phanar is so small, our tour didn't take long.
It had just ended when a Turkish policeman informed the patriarch that there was a threat on his life. It turned out to be nothing, but church officials say previous threats have been serious enough that the Phanar is surrounded by barbed wire and cameras and the patriarch has 24-hour protection.
"I think a lot of people would want to know, your all holiness, why the leader of so many millions of Orthodox Christians in the world lives in a country that is 99 percent Muslim?" Simon asked.
"Because we are here before this country becomes a Muslim country, much earlier. Since ever. Since the very beginning," the patriarch explained.
Asked since the beginning of what, Patriarch Bartholomew said, "Of the foundation of our church, of the Church of Constantinople."
And in the beginning Istanbul was called Constantinople, the ancient city on the Bosporus where east meets west. The city's skyline is dominated by minarets; at Friday prayers, the mosques are teeming.
But the city's richest and most renowned Christian churches are museums today, Meccas for tourists, not for worshipers. There's the Chora Church, with its fresco of Jesus whose eyes seem to go right through you, and he Hagia Sophia, the first great church in Christendom and an architectural wonder built 1,000 years before Saint Peter's Basilica in Rome, and for centuries the most important church in the Christian world.