Shouts of “Papa Leo” and “Viva il Papa” (Long live the pope ) have reverberated throughout Istanbul’s Cathedral of the Holy Spirit, accompanied by enthusiastic cheering and applause, as Pope Leo XIV arrived to commence his first trip in Turkiye.

The pope led a prayer service with Turkish Catholic clergy and religious sisters before turning to the primary purpose of his visit—the first of his pontificate. He will mark the 1,700th anniversary of a pivotal moment in Christianity: the 325 AD Council of Nicaea, where bishops formulated the Nicene Creed, a declaration of faith still recited by millions of Christians worldwide. The creed professes belief in one God, acknowledges Jesus as the human son of God, and the Holy Spirit as an equal person in the Trinity.

This historic council occurred when Eastern and Western churches remained unified, long before the Great Schism of 1054 divided them primarily over papal authority. Nevertheless, the Nicene Creed continues to be embraced by Catholic, Orthodox and most traditional Protestant denominations, serving as a rare point of consensus and the most universally accepted creed in Christendom.

Consequently, celebrating its origins represents a significant milestone in the centuries-long effort to reunify Christianity.

Addressing the congregation at the cathedral, Leo emphasised that the creed was not merely a doctrinal statement 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.

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Leo arrived in Turkiye on Thursday, promoting a message of peace during his meeting with Turkish President Recep Tayyip Erdogan. He will carry this same message to Lebanon on Sunday during the second and final leg of his journey as history’s first American pope.

At Tuesday’s cathedral service, Leo encouraged Turkiye’s small Catholic community of about 33,000 people in a predominantly Sunni Muslim nation of 85 million. He particularly praised the church’s outreach to migrants and newcomers.

“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.”

His message resonated strongly with the diverse crowd gathered outside, which reflected the multinational character of Turkiye’s Catholic Church.

“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, acknowledged the ecumenical importance of Leo’s visit.

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