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Leo presided over a prayer with Catholic clergy and nuns before taking part in the key reason for his visit. He will commemorate the 1,700th anniversary of one of the in Christianity — the AD 325 gathering of bishops that produced the Nicaean Creed, a statement of faith that millions of Christians still recite today.

The gathering happened at a time when the Eastern and Western churches were still united. They split in the Great Schism of 1054, a divide precipitated largely by disagreements over the primacy of the pope. But even today, Catholic, Orthodox and most historic Protestant groups accept the , making it a point of agreement and the most widely accepted creed in Christendom.

As a result, celebrating its origins at the site of its creation with the spiritual leaders of the Catholic and Orthodox churches and other Christian representatives marked a historic moment in the centuries-old quest to re-unite all Christians.

Speaking at the cathedral, Leo said the creed wasn't merely a doctrinal formula, but the “essential core of the Christian faith."

“Therefore, its development is organic, akin to that of a living reality, gradually bringing to light and expressing more fully the essential heart of the faith,” he said.

The anniversary commemoration will take place in Iznik, gathering, about 150 kilometers (93 miles) from Istanbul. Presiding with Leo will be Ecumenical Patriarch Bartholomew, the spiritual leader of the world's Orthodox Christians.

Christians are a minority in predominantly Sunni Muslim Turkey, and ahead of the prayer in Iznik, around 20 members of a small Turkish Islamic party staged a brief protest, saying the encounter posed a threat to Turkey’s sovereignty and national identity.

Under a heavy police presence, Mehmet Kaygusuz, a member of the New Welfare Party, read a statement denouncing what he said were efforts to establish a “Vatican-like Greek Orthodox state” in Turkey. The group dispersed peacefully shortly after.

Leo on Thursday, emphasizing a message of peace as he met with Turkish President Recep Tayyip Erdogan. It's a message he'll bring to Lebanon on Sunday, the second and final leg of the trip for history's first American pope.

At the cathedral on Friday, Leo sought to encourage Turkey's tiny Catholic community, which numbers around 33,000 in a nation of 85 million, most of whom are Sunni Muslims. He praised especially the church’s work with migrants and newcomers to Turkey.

“The logic of littleness is the church’s true strength,” Leo told them in English. “The significant presence of migrants and refugees in this country presents the church with the challenge of welcoming and serving some of the most vulnerable.”

It was a message that resonated with the crowd outside, which reflected the multinational face of the Catholic Church in Turkey.

“With all my heart I am so happy,” said Debora Martina Da Silva, a political science student from Guinea Bissau.

Mateusz Zajdecki, a 21-year-old from Szczecin, Poland, recognized the ecumenical significance of Leo’s visit.

“I think it is important for Turkey to (be) united at one table, eucharistic table, and to pray together to one father who is in heaven,” Zajdecki said.

Associated Press religion coverage receives support through the AP’s with The Conversation US, with funding from Lilly Endowment Inc. The AP is solely responsible for this content.