Pope Francis' celebration of an ecumenical vespers at the Basilica of St. Paul Outside the Walls Jan. 25, ending the Christian unity week, will also mark the 55th anniversary of the announcement of the Second Vatican Council.

On Jan. 25, 1959, Bl. John XXIII proclaimed his intention to call an ecumenical council at the end of a Vespers celebration held in the basilica, that concluded the Week of Prayer for Christian Unity.

The decision by the late Pope renewed ecumenism: the following year, he established the Pontifical Council for Promoting Christian Unity, and his successor, Paul VI, met with the Eastern Orthodox Patriarch of Constantinople in 1964.

Pope Francis will follow in these footsteps, first with the celebration of ecumenical Vespers, and in May, he too will meet with the Patriarch of Constantinople.

He has consistently referred to himself as "Bishop of Rome" in favor of "Pope," seen as a conciliatory gesture to the Orthodox, who object to some expressions of papal primacy.

The Church unity week was established as the Octave of Christian Unity by Fr. Lewis Wattson in 1899, who at the time was an Anglican priest. The previous year, he had co-founded an Anglican Franciscan religious community, the Society of the Atonement.

The Octave is observed between Jan. 18, the feast of Chair of St. Peter at Rome, and Jan. 25, the feast of the Conversion of St. Paul.

The Society of the Atonement sought to join the Catholic Church, and in 1909 its members were corporately received into the Church.

Fr. Wattson's church unity octave was indulgenced by both Benedict XV and Pius XII.

In the 1930s, Fr. Wattson's conception of the octave was largely supplanted by that of Fr. Paul Couturier, who saw the week as a prayerful request for reconciliation among all the baptized, believing, "we do not pray for the conversion to a Church, but for the conversion to Christ."

This new impetus led further ecclesial communities to join the week.

The conclusion of the week for Christian unity is traditionally held at St. Paul's Outside the Walls, since it is the feast of St. Paul's conversion which draws the week to its end.

The basilica has long been entrusted to the Benedictines, and its link with St. Paul has made it known for its ecumenical commitment. At the beginning of his pontificate, Benedict XVI gave the basilica's Benedictines a particular mandate to work for ecumenism.