Chapter 18 — Part II: The Sacraments — The Holy Eucharist
Section 38. Water Should Be Mixed With The Wine — With the wine, however, the Church of God has always mingled water. First, because Christ the Lord did so, as is proved by the authority of Councils and the testimony of St. Cyprian; next, because by this mixture is renewed the recollection of the blood and water that issued from His side. Waters, also, as we read in the Apocalypse, signify the people; and hence, water mixed with the wine signifies the union of the faithful with Christ their Head. This rite, derived as it is from Apostolic tradition, the Catholic Church has always observed.