Where Christians forged a common creed, Pope Leo calls to end 'scandal of divisions'

"We must strongly reject the use of religion for justifying war, violence or any form of fundamentalism or fanaticism," he added. "Instead, the paths to follow are those of fraternal encounter, dialogue and cooperation."

Looking out over the lake, Leo and Bartholomew stood before the ruins of the ancient Basilica of St. Neophytos, believed by some to have been the venue for the Council of Nicaea. The basilica's sunken foundations were rediscovered in Lake Iznik in 2014 from aerial photographs and later excavated.

In welcoming the pope and other delegations, Bartholomew said that the power of Nicaea "does not reside in what passes away, but in what endures forever."

"Having the fervor of the faith of Nicaea burning in our hearts, let us run the course of Christian unity that is set before us," he said.

Both Leo and Bartholomew have expressed their desire to achieve full communion between the Catholic and Orthodox Churches, though questions around doctrine and nontheological issues remain obstacles.

At Iznik, Leo framed the council as one "of fundamental importance in the journey that Christians are making toward full communion."

The pope's decision to mark the council's anniversary in Turkey, a trip originally planned for Pope Francis, signals the emphasis on unity he has developed early in his pontificate.

Turkey's tiny Catholic community

While Leo's first day in Turkey was dominated by meetings with political leaders in Ankara, including Turkish President Recep Tayyip Erdogan and an address to government officials, his second day shifted the focus to his role as a pastor. Before traveling to Iznik, Leo met representatives of Turkey's minuscule Catholic community, visited a nursing home run by the Little Sisters of the Poor and had a private meeting with Turkey's Chief Rabbi David Sevi.

At Istanbul's Cathedral of the Holy Spirit, tucked behind an unassuming building front in the heart of Turkey's largest city, Leo urged Turkey's Catholics to not "look with resignation at how small the Catholic Church has become numerically."

Only 33,000 Catholics are present in Turkey, making up just 0.04% of the population, according to Vatican statistics.

The  church's true strength lies in the "logic of littleness" chosen by God, Leo said.

"It does not lie in her resources or structures, nor do the fruits of her mission depend on numbers, economic power or social influence."

Though the pope's first meeting with a Catholic community abroad was nowhere near the size it would be in a country with a large Christian population, the more than 550 people who filled the cathedral made their fervor felt, cheering and chanting as Leo entered the church.

The pope, usually calm and mild-mannered, was visibly moved. His eyes welled with tears, and his voice caught as he offered his opening blessing.

In his remarks, Leo drew a direct line between the council whose anniversary he would commemorate later that day and the challenges currently facing Christians.

While the Council of Nicaea rejected the heresy of denying Jesus' divinity known as Arianism, the pope warned of a "new Arianism present in today's culture and sometimes even among believers," in which Jesus is "reduced to a great historical figure, a wise teacher, or a prophet who fought for justice — nothing more."

"Nicaea reminds us that Jesus Christ is not a figure of the past; he is the Son of God present among us, guiding history toward the future promised by God," he said.

Yet Leo was quick to note that historical formulas that express faith "are always partial and provisional and can change as doctrine is more deeply understood."

He recalled how St. John Henry Newman, whom the pope named a doctor of the church on Nov. 1, "insisted on the development of Christian doctrine, because doctrine is not an abstract, static idea, but reflects the very mystery of Christ."

Despite its small size, Leo also highlighted the church's ministries in Turkey, especially its pastoral outreach to refugees and migrants — a major reality in Turkey, which hosts millions of displaced people from Syria and elsewhere.

Ministering amid a Muslim majority

For the small Catholic community living in overwhelmingly Muslim Turkey, Leo's visit carried both encouragement and visibility.

Sr. Imurere Irene, a missionary of the Society of Mary from Rwanda who has served in Turkey for two years, told the National Catholic Reporter that the Christian community's small size fosters a shared sense of witness.

Christians of all creeds in Turkey "are united, we try to help each other," she said while awaiting the pope's arrival in the cathedral. "We try, through our way of life, to show the Muslim [community] what Christianity is and to be witness[es] of Christ."

Franciscan Fr. Paolo Pugliese, an Italian who has been ministering in Turkey for 10 years, said the pope's visit has generally been perceived positively by Turkey's Muslim community, "especially because of the commitment of the pope in spreading peace, in talking about peace."

Both Francis and Leo "have been very clear in talking about peace for Gaza, and this was quite a clear sign for Muslim countries that allowed them to feel their closeness," he told NCR.

The war in Gaza will linger over the next leg of Leo's trip to Lebanon, which has continued to be struck by Israeli air raids in recent weeks.

Though Lebanon's Catholic community is sizable, the Muslim community will be listening attentively to how the leader of the world's largest Christian church calls for peace in a region consumed by interreligious tensions and conflict.