2 This faith, uninterruptedly professed by the Church, needs to be constantly reawakened and deepened in the consciousness of the People of God. In the course of the last hundred years this has been done several times: by Leo XIII, who published the Encyclical Epistle Divinum Illud Munus (1897) entirely devoted to the Holy Spirit; by Pius XII, who in the Encyclical Letter Mystici Corporis (1943) spoke of the Holy Spirit as the vital principle of the Church, in which he works in union with the Head of the Mystical Body, Christ ; at the Second Vatican Ecumenical Council which brought out the need for a new study of the doctrine on the Holy Spirit, as Paul VI emphasized: "The Christology and particularly the ecclesiology of the Council must be succeeded by a new study of and devotion to the Holy Spirit, precisely as the indispensable complement to the teaching of the Council." In our own age, then, we are called anew by the ever ancient and ever new faith of the Church, to draw near to the Holy Spirit as the giver of life. In this we are helped and stimulated also by the heritage we share with the Oriental Churches, which have jealously guarded the extraordinary riches of the teachings of the Fathers on the Holy Spirit. For this reason too we can say that one of the most important ecclesial events of recent years has been the Sixteenth Centenary of the First Council of Constantinople, celebrated simultaneously in Constantinople and Rome on the Solemnity of Pentecost in 1981. The Holy Spirit was then better seen, through a meditation on the mystery of the Church, as the one who points out the ways leading to the union of Christians, indeed as the supreme source of this unity, which comes from God himself and to which St. Paul gave a particular expression in the words which are frequently used to begin the Eucharistic liturgy: "The grace of our Lord Jesus Christ and the love of God and the fellowship of the Holy Spirit be with you all." In a certain sense, my previous Encyclicals Redemptor Hominis and Dives in Misericordia took their origin and inspiration from this exhortation, celebrating as they do the event of our salvation accomplished in the Son, sent by the Father into the world "that the world might be saved through him" and "every tongue confess that Jesus Christ is Lord, to the glory of God the Father." From this exhortation now comes the present Encyclical on the Holy Spirit, who proceeds from the Father and the Son; with the Father and the Son he is adored and glorified: a divine Person, he is at the center of the Christian faith and is the source and dynamic power of the Church's renewal. The Encyclical has been drawn from the heart of the heritage of the Council. For the Conciliar texts, thanks to their teaching on the Church in herself and the Church in the world, move us to penetrate ever deeper into the Trinitarian mystery of God himself, through the Gospels, the Fathers and the liturgy: to the Father, through Christ, in the Holy Spirit. In this way the Church is also responding to certain deep desires which she believes she can discern in people's hearts today: a fresh discovery of God in his transcendent reality as the infinite Spirit, just as Jesus presents him to the Samaritan woman; the need to adore him "in spirit and truth" ; the hope of finding in him the secret of love and the power of a "new creation" : yes, precisely the giver of life. The Church feels herself called to this mission of proclaiming the Spirit, while together with the human family she approaches the end of the second Millennium after Christ. Against the background of a heaven and earth which will "pass away," she knows well that "the words which will not pass away" acquire a particular eloquence. They are the words of Christ about the Holy Spirit, the inexhaustible source of the "water welling up to eternal life," as truth and saving grace. Upon these words she wishes to reflect, to these words she wishes to call the attention of believers and of all people, as she prepares to celebrate- as will be said later on-the great Jubilee which will mark the passage from the second to the third Christian Millennium. Naturally, the considerations that follow do not aim to explore exhaustively the extremely rich doctrine on the Holy Spirit, nor to favor any particular solution of questions which are still open. Their main purpose is to develop in the Church the awareness that "she is compelled by the Holy Spirit to do her part towards the full realization of the will of God, who has established Christ as the source of salvation for the whole world."
Source: Dominum et Vivificantem (Vatican.va)