Mandate

The mandate for the Office comes directly from the Archbishop.

That mandate finds it roots in the documents of the Second Vatican Council, which urge these efforts.

The restoration of unity among all Christians is one of the principal concerns of the Second Vatican Council. Christ the Lord founded one Church and one Church only. However, many Christian communions present themselves to men as the true inheritors of Jesus Christ; all indeed profess to be followers of the Lord but differ in mind and go their different ways, as if Christ Himself were divided.  Such division openly contradicts the will of Christ, scandalizes the world, and damages the holy cause of preaching the Gospel to every creature.

But the Lord of Ages wisely and patiently follows out the plan of grace on our behalf, sinners that we are. In recent times more than ever before, He has been rousing divided Christians to remorse over their divisions and to a longing for unity. Everywhere large numbers have felt the impulse of this grace, and among our separated brethren also there increases from day to day the movement, fostered by the grace of the Holy Spirit, for the restoration of unity among all Christians. This movement toward unity is called "ecumenical." Those belong to it who invoke the Triune God and confess Jesus as Lord and Savior, doing this not merely as individuals but also as corporate bodies. For almost everyone regards the body in which he has heard the Gospel as his Church and indeed, God's Church. All however, though in different ways, long for the one visible Church of God, a Church truly universal and set forth into the world that the world may be converted to the Gospel and so be saved, to the glory of God.
The Sacred Council gladly notes all this. It has already declared its teaching on the Church, and now, moved by a desire for the restoration of unity among all the followers of Christ, it wishes to set before all Catholics the ways and means by which they too can respond to this grace and to this divine call. (Unitatis Redintegratio, 1)

The attainment of union is the concern of the whole Church, faithful and shepherds alike. This concern extends to everyone, according to his talent, whether it be exercised in his daily Christian life or in his theological and historical research. This concern itself reveals already to some extent the bond of brotherhood between all Christians and it helps toward that full and perfect unity which God in His kindness wills. (Unitatis Redintegratio, 5)

The Catholic Church rejects nothing that is true and holy in these [non-Christian] religions….  She regards with sincere reverence those ways of conduct and of life, those precepts and teachings which, though differing in many aspects from the ones she holds and sets forth, nonetheless often reflect a ray of that Truth which enlightens all men. Indeed, she proclaims, and ever must proclaim Christ "the way, the truth, and the life" (John 14:6), in whom men may find the fullness of religious life, in whom God has reconciled all things to Himself.

The Church, therefore, exhorts her [children], that through dialogue and collaboration with the followers of other religions, carried out with prudence and love and in witness to the Christian faith and life, they recognize, preserve and promote the good things, spiritual and moral, as well as the socio-cultural values found among these [people].  (Nostra Aetate, 2)