Encyclical ’Ut Unum Sint’ ("That they might be one")

Like many encyclicals, this one derives its title from its "incipit" or first few words. These words are taken from the prayer of Jesus "at the hour of his Passion" in the Vulgate translation of the Gospel according to John. The document deals with the Catholic Church's relations with the Eastern Orthodox Church and other Christian churches. It reiterates that unity of these two sui juris churches is essential, as well as further dialogue and unity with the Protestant churches. This document shows that the Catholic Church is officially committed to unity. It has become a common piece of study in ecumenical classes.

Although it builds on the Second Vatican Council's decree, Unitatis redintegratio, and pays testimony to the work of Popes Paul VI and John Paul I, Ut unum sint was the first encyclical ever devoted exclusively to the ecumenical imperative. In this groundbreaking exercise of the papal magisterium, Pope John Paul II affirmed that the ecumenical commitment made at Vatican II was irreversible. He taught his fellow Catholics that the quest for Christian unity ought to be sustained both internationally and in the local churches.