Catechism of the Catholic Church §1108 — excerpt
In every liturgical action the Holy Spirit is sent in order to bring us into communion with Christ and so to form his Body. the Holy Spirit is like the sap of the Father's vine which bears fruit on its branches.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.