- Pope Francis pushes for reunification with Orthodox Christians
- Pontiff to meet with youths before departing
- During visit, Francis calls for religious freedom and tolerance to counter extremism
- Pope: Fanaticism and fundamentalism need to be countered by those of faith
Christianity is a religion frayed over the centuries into many denominations. On Sunday, Pope Francis and the head of a major one vowed to heal one of the church's oldest splits.
The Great Schism of 1054 separated the Catholic and Eastern Orthodox churches. Both profess similar doctrine and worship in similar ways, but a millennium ago, Eastern Orthodoxy rejected the ultimate authority of the Pope.
Francis and Patriarch Bartholomew I -- the spiritual leader of some 300 million Orthodox Christians worldwide -- signed a declaration on Sunday committing to unity between the two churches.
Together, they celebrated a divine liturgy at the Patriarchal Church of St. George in Istanbul on Francis' last day in the predominantly Muslim nation -- his first trip there.
Somber intonations and hymns reverberated through the church as the two spiritual leaders prayed and offered reflections and Francis tried to allay worries.
Unity, not submission
Neither church will submit to nor assimilate the other, the Pope said in his homily, but they will work "towards the restoration of full Communion."
"I want to assure each one of you here that, to reach the desired goal of full unity, the Catholic Church does not intend to impose any conditions except that of the shared profession of faith," Francis said.
He was at St. George's at the Patriarch's invitation, which he extended shortly after Francis' inauguration.
Sunday's celebrations marked the feast of St. Andrew, the founder of the Eastern Church. He was also the older brother of St. Peter, the founding father of the Catholic Church .
'Blood of martyrdom'
In his part of the call for unity, the Orthodox leader alluded to attacks by radical Islamists on Christians.
"We no longer have the luxury of isolated action," Bartholomew said. "The modern persecutors of Christians do not ask which church their victims belong to. The unity that concerns us is regrettably already occurring in certain regions of the world through the blood of martyrdom."
Over the past decade, first al Qaeda and then ISIS have forced Christians to flee Iraq and Syria.
Far fewer Christians remain in areas under ISIS control. Some rely on their faith in God; others pay terrorists a protection tax. Growing numbers, especially from Syria, have ended up in Turkey.The nation is reeling from the influx of around 1.5 million Syrian refugees, the vast majority Muslim.
Religion is also splitting Turkey. Opponents fear its conservative government will weaken the secular identity of the post-Ottoman Empire.
Instability rages at its border, with ISIS and its leader, Abu Bakr al-Baghdadi, announcing their intention to take over large swaths of territory for their self-proclaimed Islamic State.
Fourth pope in Turkey
Francis is the fourth pope to travel to Turkey, following in the footsteps of Paul VI, John Paul II and Benedict XVI.
The Pope was also expected to meet with young people from the Salesian community from Turkey, the Middle East and Africa before heading to the airport later Sunday. Salesians, the second largest Catholic order, help orphans and at-risk children, according to their website.
On Friday, the first day of his visit, Francis called for religious tolerance and dialogue to counter extremism in the Middle East, as he met with Turkey's leaders.
"Fanaticism and fundamentalism, as well as irrational fears, which foster misunderstanding and discrimination, need to be countered by the solidarity of all believers," he said.
The world is morally obligated to help Turkey care for the great number of refugees it has taken in, Francis said.
Erdogan criticizes Islamophobia
In an address given in Francis' presence, Turkish President Recep Tayyip Erdogan highlighted what he said was a disturbing trend of increased racism and Islamophobia in the West, while Islamist extremism wracks parts of the Middle East.
The world must come together if it wants to combat terrorism, he said.
The Turkish leader also suggested the West should not turn a blind eye to abuses committed by Syrian President Bashar al-Assad while it pursues ISIS.
The timing of the Pope's visit, in this period of unrest, is extremely significant, Erdogan added, voicing hope that it would lead to an "auspicious era" of improved relations in the world.