Down a narrow side street, in a district of Istanbul on the Golden Horn well off the beaten tourist track, sit the relatively modest headquarters of the "first among equals" of the world's estimated 300 million Orthodox Christian believers.
From there Bartholomew I, known as Archbishop of Constantinople, New Rome and Ecumenical Patriarch, serves in an office that dates back to the early days of the Byzantine Empire, over a millennium before the Ottoman conquest of the city in 1493.
His office of Ecumenical Patriarch has remained in place through all the historical convulsions since, its spiritual importance now out of all proportion to his relatively modest base in Istanbul.
Bartholomew's significance will be underlined again on Saturday and Sunday when he meets Pope Francis on his first visit to Turkey, the latest step in narrowing the schism between the Orthodox and Roman Catholic churches that dates back to 1054.
Under Bartholomew, in office since 1991, the archbishop's role has taken on a new importance going even beyond the bounds of Orthodoxy.
He has won praise for seeking to build bridges with the Catholic faith as well as Islam and also winning accolades for his commitment to environmental activism.
- 'A really fine man' -
Last weekend, visiting US Vice President Joe Biden made a point of finding time to hold a lengthy meeting with Bartholomew at the patriarchate.
"This is a really fine man!" Biden declared.
Bartholomew is the primus inter pares (first among equals) of Orthodox churches across the world, including Greek, Russian, Serbian and Romanian. His degree of influence varies, but many consider him to be the spiritual head of the entire Orthodox faith.
He remains known as Archbishop of Constantinople, in a throwback to the former Byzantine name of the city, which was only officially renamed as Istanbul in the 1920s after the fall of the Ottoman Empire.
A holder of the US Congressional Gold Medal bestowed on him in 1997, Bartholomew was born in 1940 on the island of Gokceada in western Turkey as Dimitrios Archontonis.
But his authority has not spared him from an uphill battle for greater rights inside Turkey.
The Turkish authorities do not recognise Bartholomew as the Ecumenical Patriarch but just as the leader of Turkey’s tiny remaining Greek Orthodox minority of just 2,500 people.
His exalted titles in Greek carry no weight for Turkish officials who know him as the Fener Rum Ortodoks Patrigi (Greek Orthodox Patriarch of Fener) after the Istanbul district where the patriarchate is located.
His position is subject to strict rules by the Turkish authorities, most notably that its holder must be a Turkish citizen.
The Turkish authorities have also not approved repeated requests to reopen the patriarchate's seminary on the island of Heybeliada (Halki in Greek) off Istanbul, creating concerns about where Bartholomew's successor will be found.
Bartholomew angered the Turkish authorities in 2009 when he told a US television network he felt "second class" and even "crucified" in Turkey.
Then foreign minister Ahmet Davutoglu, now prime minister, described the remarks as "extremely unfortunate".
Turkish police in 2013 said they had uncovered a plot to assassinate Bartholomew but his office said the patriarch did not believe it was something serious.
- 'Yet another big step' -
His term in office has also been marked by rocky relations with the Russian Orthodox Church, which gives short shrift to the idea he is the spiritual leader of Orthodox believers.
When Russian President Vladimir Putin in 2013 held an unprecedented gathering of the world's Orthodox leaders in the Kremlin, Bartholomew did not attend and instead sent a lower-ranking cleric.
But the focus of his meeting with Pope Francis will be the rapprochement with the Catholic Church, a process that began in 1964 with the embrace between Pope Paul VI and Ecumenical Patriarch Athenagoras, the first such meeting since the 15th century.
Bartholomew also visited the Vatican for the inaugural mass of Pope Francis, the first time in history that an Ecumenical Patriarch had attended the installation of a Pope.
"We are eagerly awaiting the visit of our brother, Pope Francis," Bartholomew said ahead of the visit. "It will be yet another significant step in our positive relations as sister Churches."