Catechism of the Catholic Church §1375 — excerpt
It is by the conversion of the bread and wine into Christ's body and blood that Christ becomes present in this sacrament. the Church Fathers strongly affirmed the faith of the Church in the efficacy of the Word of Christ and of the action of the Holy Spirit to bring about this conversion.
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.
Whole chapters cited: Jude 380
Also cites: Jude 1:6: PG 49 · St. Ambrose, De myst. 9, 50 · 52: PL 16, 405-407