Catechism of the Catholic Church §1410 — excerpt
It is Christ himself, the eternal high priest of the New Covenant who, acting through the ministry of the priests, offers the Eucharistic sacrifice. and it is the same Christ, really present under the species of bread and wine, who is the offering of the Eucharistic sacrifice.
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.