Christian churches and communities across the nation will celebrate their unity this week with prayer meetings, services, and worship events.

As part of the international Week of Prayer for Christian Unity, groups such as the National Council of Churches are making available resources for local churches to use as they plan out various activities.

Resources for the week include an introduction to the theme, “You are witnesses of these things;” suggestion for an ecumenical celebration that local churches are encouraged to adapt within their own particular liturgical, social and cultural contexts; biblical reflections and prayers for the "eight days;" and an overview of the ecumenical situation in Scotland, from where the inspiration for the event is said to have originated.

The Week of Prayer for Christian Unity, according to Pope Benedict XVI, is a time to "revive the ecumenical spirit."

The annual event "constitutes, for believers in Christ, a propitious time to revive the ecumenical spirit, to meet each other, to get to know each other, to pray and reflect together," the pontiff said Sunday during his pre-prayer address to those gathered in St. Peter's Square.

The main sponsors of the Week of Prayer are the Roman Catholic Church and the World Council of Churches, which brings together 349 churches, denominations and church fellowships in more than 110 countries and territories throughout the world.

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The initiative officially began in 1964 during the Second Vatican Council with a call for prayer on the soul of the ecumenical movement.

It has since spread throughout all Christian denominations and faith bodies, many of which (particularly in the northern hemisphere) celebrate it each year between Jan. 18 and Jan. 25, while others (particularly in the southern hemisphere) observe it at Pentecost.

In his address Sunday, Benedict explained that the theme this year from Luke's Gospel "echoes the words of the risen Jesus to the apostles: 'You are witnesses of these things.'"

"Our proclamation of the Gospel of Jesus will be much more credible and effective the more that we are united in his love, as true brothers," the pope affirmed. "Thus, I invite parishes, religious communities, ecclesial movements and associations to pray unceasingly, in a special way during Eucharistic Celebrations, for the complete unity of Christians."

This year's theme was chosen in Scotland, where churches have been preparing to celebrate the anniversary of the 1910 World Mission Conference, which marked the beginnings of the modern ecumenical movement.

In the United States, materials for the celebration of the Week of Prayer are the work of Graymoor Ecumenical & Interreligious Institute in collaboration with the Commission on Faith & Order of the National Council of Churches and the Roman Catholic Bishops' Commission for Ecumenical and Interreligious Affairs.