Catechism of the Catholic Church §766 — excerpt
The Church is born primarily of Christ's total self-giving for our salvation, anticipated in the institution of the Eucharist and fulfilled on the cross. "The origin and growth of the Church are symbolized by the blood and water which flowed from the open side of the crucified Jesus."
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.
Also cites: LG 3 · SC 5 · St. Ambrose, In Luc. 2, 85-89 PL 15,1666-1668