Catechism of the Catholic Church
1104 Christian liturgy not only recalls the events that saved us but actualizes them, makes them present. The Paschal mystery of Christ is celebrated, not repeated. It is the celebrations that are repeated, and in each celebration there is an outpouring of the Holy Spirit that makes the unique mystery present.
1105 The Epiclesis ("invocation upon") is the intercession in which the priest begs the Father to send the Holy Spirit, the Sanctifier, so that the offerings may become the body and blood of Christ and that the faithful by receiving them, may themselves become a living offering to God. 23
1106 Together with the anamnesis, the epiclesis is at the heart of each sacramental celebration, most especially of the Eucharist:
You ask how the bread becomes the Body of Christ, and the wine . . . the Blood of Christ I shall tell you: the Holy Spirit comes upon them and accomplishes what surpasses every word and thought . . . Let it be enough for you to understand that it is by the Holy Spirit, just as it was of the Holy Virgin and by the Holy Spirit that the Lord, through and in himself, took flesh. 24
1107 The Holy Spirit's transforming power in the liturgy hastens the coming of the kingdom and the consummation of the mystery of salvation. While we wait in hope he causes us really to anticipate the fullness of communion with the Holy Trinity. Sent by the Father who hears the epiclesis of the Church, the Spirit gives life to those who accept him and is, even now, the "guarantee" of their inheritance. 25
English Translation of the Cathechism of the Catholic Church for the United States of America © 1997, United States Catholic Conference, Inc.