The service was part of celebrations marking the 1,000th anniversary of the founding of the Camaldolese branch of the Benedictine order. Camaldoli monks and nuns live and pray at the Church of St. Gregory and have an active program of ecumenical contacts.
"We hope that the sign of our presence here together in front of the holy altar, where Gregory himself celebrated the eucharistic sacrifice, will remain not only as a reminder of our fraternal encounter, but also as a stimulus for all the faithful -- both Catholic and Anglican -- encouraging them ... to renew their commitment to pray constantly and to work for unity, and to live fully in accordance with the 'ut unum sint' (that all may be one) that Jesus addressed to the Father," Pope Benedict said during the evening prayer service.
Faith is a gift of God, but it requires a response, the Pope said. "It requires the commitment to be reclothed in Christ's sentiments: tenderness, goodness, humility, meekness, magnanimity, mutual forgiveness and, above all, as a synthesis and a crown, 'agape' -- the love that God has given us through Jesus, the love that the Holy Spirit has poured into our hearts," he said.
Camaldoli monks and nuns -- wearing hooded white robes -- were joined by cardinals, Anglican and Catholic faithful and representatives of other Christian communities in Rome for the prayer service.
As the Pope and archbishop arrived at St. Gregory, they also were greeted by dozens of members of the Missionaries of Charity, who have a convent and a shelter for the homeless next door.
The Pope and the archbishop of Canterbury held private talks in the morning March 10 at the Vatican.
Archbishop Williams told Vatican Radio that he and the Pope spoke about the situation of Christians in the Middle East "and about our shared sense of deep anxiety and frustration and uncertainty about what the future holds there."
He said they also spoke about Pope Benedict's invitation to Archbishop Williams to address October's world Synod of Bishops on the new evangelization.
"I'm being invited to give some theological reflections on the nature of mission, the nature of evangelization, and I'm extremely honored to be invited to do this," he told Vatican Radio.
"I hope that it's a sign that we can work together on evangelization in Europe," the archbishop said. "It's disastrous if any one church tries to go it alone here and tries to assume that it and it alone has the key," because reviving the Christian faith in Europe requires as many and "as deep resources as we can find."
Archbishop Williams' homily at the evening prayer service with the Pope focused on how the Camaldolese efforts to balance solitude and community life teach the virtues individual Christians and Christian communities need to accept each other, work together and witness the Gospel to all.
Anglicanism and Roman Catholicism, he said, both are committed to "a vision of the restoration of full sacramental communion, of a eucharistic life that is fully visible, and thus a witness that is fully credible, so that a confused and tormented world may enter into the welcome and transforming light of Christ."
But Catholic-Anglican unity is imperfect, at least in part because Catholics and Anglicans have an "unstable and incomplete" recognition of one another as the body of Christ, Archbishop Williams said. "Without such ultimate recognition we are not yet fully free to share the transforming power of the Gospel" within the Christian community and in the world.
He told Vatican Radio that Anglicans and Roman Catholics "can become so fixated" on issues of authority and church structure "that we can forget the gift of baptism and the gift of one another in baptism," which are the true basis of unity.
Archbishop Williams spoke again March 11 at St. Gregory, giving a conference on ecumenism and monasticism along with Father Robert Hale, prior of the New Camaldoli Monastery in Big Sur, Calif.
He said all Christians can learn from the way the Camaldolese try to balance solitude and community life. "Christian solitude is the way in which we allow God to challenge and overcome our individualism," leading to a community life where Christians recognize their unity is based on faith and not on ties of blood or nationality.
"One of the hardest, yet most important, lessons the different Christian communities today have to learn is that they cannot live without each other and that no single one of them in isolation possesses the entirety of the Gospel," he said. In their divisions, Christian communities have developed different spiritual gifts and traditions, which should be shared to build up all communities.