These words, known as the “Testament of Cardinal Mercier,” are the inspiration for the annual week of prayer for Christian unity. Celebrated around the world, the theme and text that are intended to be the focus of study and prayer are selected by an international committee composed of members of the World Council of Churches and the Pontifical Council for the Promotion of Christian Unity; national and regional councils work to adapt and add to these resources as appropriate to their local context.

The traditional date for the week—or more specifically an octave, or eight days—is 18-25 January. These are the dates that were proposed in 1908 by a priest of the Episcopal Church, Paul Wattson, who suggested that the days between the feasts of St Peter and St Paul would be particularly apt. In the southern hemisphere, where January falls during the summer vacation, churches often observe the week around Pentecost, another festival symbolic of unity.