Four bishops, two of whom retire this year, are to be co-presiders at a Week of Prayer for Christian Unity service tonight at Assumption College. Also expected to participate are a retired bishop, clergy, religious and laity. Anyone interested is welcome to attend the prayer service at 7 p.m. in the Chapel of the Holy Spirit, hosted by the president’s office and campus ministry said Father Dinh Vo Tran, an Augustinian of the Assumption and a campus minister. Campus Ministry worked with a committee of representatives from different churches and from the college to prepare the service, at which Bishop McManus will preach, he said. The other co-presiders are Bishop Gordon P. Scruton, of the Episcopal Diocese of Western Massachusetts, and Bishop Margaret Payne, of the New England Synod of the Evangelical Lutheran Church in America, both of whom are retiring this year, and Metropolitan Methodios, of the Greek Orthodox Metropolis of Boston, Father Vo Tran said. Bishop Reilly is also to participate. Francesco C. Cesareo, Assumption College’s president, will give a welcome and acknowledge the retiring bishops, Father Vo Tran said. The Week of Prayer for Christian Unity is traditionally celebrated between the Feast of the Chair of St. Peter (formerly on January 18) and the Feast of the Conversion of St. Paul the Apostle on January 25. This year’s theme is “We will all be changed by the victory of our Lord Jesus Christ,” from I Cor. 15:51-58. In keeping with the theme, this year’s donations of money and non-perishable food will be given to the Victory Center, 371 Pleasant St. in Worcester, said Father Vo Tran. The center is a ministry of Victory in Christ Church, Worcester South Foursquare Church. All present will be invited to join in a “Ritual of Unity,” during which they will bring their donations toward the altar. Another ritual will be a veneration of the Gospel and celebration of God’s Word, which Father Vo Tran said is the main thing that unites Christians. Representatives of different denominations will process forward with the Book of the Gospels, he said. The service is also to include Scripture readings, hymns and prayers.