Churches around the world kicked off on Tuesday The Week of Prayer for Christian Unity.

And ecumenical leaders are emphasizing that the week is "not just a nice occasion for friends to gather."

Rather, "it is a time to give thanks to God for the gift and promise of unity, to be renewed in our ecumenical resolve by the assurance of God’s leading, and to recommit ourselves to participate in what God is doing to overcome the barriers between God’s children," said the Rev. Dr. Michael Kinnamon, general secretary of the National Council of Churches.

Churches will be praying and celebrating under the theme "One in the apostles’ teaching, fellowship, breaking of bread and prayer," which comes from the New Testament book of Acts. It was chosen by a group of Christian leaders from Jerusalem.

Participants in the northern hemisphere are being called to not only return to the essentials of faith but to also remember the time when the church was still one.

"We do not get discouraged, even when division seems rampant, because we trust that God’s reconciling love is at work in the world, calling us, as I Corinthians puts it, to be ambassadors of that reconciliation," said Kinnamon.

The Rev. Bob Fyffe, general secretary of the ecumenical organization Churches Together in Britain and Ireland, celebrated the achievements of the ecumenical movement so far, refusing to downplay the significance of Christian unity.

Division among Christians, he pointed out, offers poor witness to the world.

And the Christian unity movement is "not about ecclesiastical coziness," he stressed.

"It is not merely that churches have become more friendly with other churches on a superficial level," he stated. "It is this movement that has helped to overcome some deep rooted enmities that have scared communities, transforming churches to be more open to each other.

"And this has not only changed them, but changed the society around them, making the lives of individuals and families more peaceful, settled, at ease with their community."

The Week of Prayer for Christian Unity is observed annually in January by churches in the northern hemisphere and at Pentecost for churches in the southern hemisphere.