"If, as Catholics and Orthodox, we wish to proclaim together the marvels of God's mercy to the whole world, we cannot continue to harbor sentiments and attitudes of rivalry, mistrust and rancor," the Pope said June 28.

The delegation, led by Greek Orthodox Metropolitan Methodios of Boston, was in Rome to represent Ecumenical Patriarch Bartholomew of Constantinople at the Pope's celebration of the feast of Sts. Peter and Paul, the patron saints of the church of Rome.

Since 1969, the patriarchs have sent delegations to the Vatican for the June 29 feast and the popes have sent a delegation to Turkey each year for the feast of St. Andrew, patron of the patriarchate.

Metropolitan Methodios is the Orthodox co-president of the North American Orthodox-Catholic Theological Consultation. Pope Francis used his presence at the Vatican as an opportunity to praise the "fruitful work" of the North American group. "Instituted more than 50 years ago, this consultation has proposed significant reflections on central theological issues for our churches, thus fostering the development of excellent relations between Catholics and Orthodox on that continent," the Pope said.

Pope Francis told the delegation that in proclaiming a Year of Mercy he wanted not only to encourage people to contemplate how merciful God is, but also to focus on ways to make the witness they give to God's mercy more effective.

"Divine mercy frees us of the burden of past conflicts and lets us be open to the future to which the Spirit is guiding us," he said.

St. Peter, who had denied Jesus, and St. Paul, who had persecuted the early Christian community, both had powerful experiences of God's forgiveness and great mercy, the Pope said. They became "tireless evangelizers and fearless witnesses to the salvation offered by God in Christ to every man and woman."

With St. Peter and St. Paul, he said, Christians are united in their experience of being forgiven and receiving God's mercy and grace.

Before the split of the churches of the East and West in the 11th century, he said, the church of Rome and the church of Constantinople were united despite differences "in the liturgical sphere, in ecclesiastical discipline and also in the manner of formulating the one revealed truth."

"However," the Pope said, "beyond the concrete shapes that our churches have taken on over time, there has always been the same experience of God's infinite love for our smallness and frailty and the same calling to bear witness to this love before the world."

"Acknowledging that the experience of God's mercy is the bond uniting us means that we must increasingly make mercy the criterion and measure of our relationship," he insisted. And that mercy must be seen in the acceptance that they will have differences.

Pope Francis also used the audience with the delegation to speak again of the importance of visiting refugees on the island of Lesbos in April with Patriarch Bartholomew and the head of the Orthodox Church of Greece.

"Seeing the despair on the faces of men, women and children uncertain of their future, listening helplessly as they related their experiences, and praying on the shore of the sea that has claimed the lives of so many innocent persons, was a tremendously moving experience," the Pope said. "A great consolation in that sad experience was the powerful spiritual and human closeness that I shared" with Patriarch Bartholomew and Archbishop Ieronymos II of Athens and all Greece.

"Led by the Holy Spirit, we are coming to realize ever more clearly that we, Catholics and Orthodox, have a shared responsibility toward those in need, based on our obedience to the one Gospel of Jesus Christ our Lord," the Pope said. "Taking up this task together is a duty linked to the very credibility of our Christian identity."

Patriarch Bartholomew, in a letter to the Pope, also recalled the importance of the visit to Lesbos and the continuing need for Christians to witness together to the values taught by the Gospel.

"The contemporary crisis of refugees and migrants has demonstrated the need for European nations to address this problem on the basis of the ancient Christian principles of fraternity and social justice," the patriarch wrote. Europe needs its Judeo-Christian roots to keep it from becoming "entirely secularized or subjected to 'economism' and various forms of fundamentalism."

"The 'culture of solidarity' nurtured by Christianity is not preserved through the progress of standards of living, the Internet and globalization," he said.

Christian action and preaching to address "the global challenges of our time will continue because they constitute a good witness for the church of Christ, serving humankind and the world," the patriarch wrote.