Chapter I.
First, he repeats what has already been said about the sacrament, and therefore that the figure of Christ himself and his sacrifice preceded in Melchizedek. Then he presents the two-fold reason why water is mixed in the chalice.
Yesterday our conversation and discussion was directed all the way to the sacraments of the holy altar. And we came to know that the figure of these sacraments preceded the time of Abraham, when the holy Melchizedek offered sacrifice (Gen. XIV, 18), having neither beginning nor end of days. Listen, man, what Paul the apostle says to the Hebrews. Where are those who say that the Son of God is from the time of Melchizedek? It is said that He has neither beginning nor end of days: if Melchizedek does not have a beginning of days, could Christ have had one (Hebr. VII, 1 et seq.)? But it is not more image than truth. Therefore, you see that he is the first and the last (Apoc. I, 8). First, because he is the author of all: last, not because he finds an end, but because he concludes everything.
Therefore, we say that a chalice and bread are placed on the altar. What is placed in the chalice? Wine. And what else? Water (On the Consecration, distinction 2, chapter On the Chalice. Exodus 17:6). But you say to me: How then did Melchizedek offer bread and wine? What is the meaning of the mixture with water? Understand the reasoning.
First of all, what is the meaning of the figure that came before the time of Moses? When the people of the Jews were thirsty and complained that they could not find water, God commanded Moses to strike the rock with his staff. Moses struck the rock, and a great amount of water came forth from the rock, as the Apostle says: 'And they drank from the spiritual rock that followed them, and the rock was Christ' (I Cor. X, 4). It was not an immovable rock that followed the people. And you drink, so that Christ may follow you. See the mystery. Moses, that is, the prophet: the staff, that is, the word of God. The priest touches the rock with the word of God, and water flows, and the people of God drink. Therefore, the priest touches, the water overflows into the chalice, it jumps into eternal life, and the people of God, who have received the grace of God, drink. So you have learned this.
Take and read another. At the time of the Lord's Passion, when the Great Sabbath was approaching, because our Lord Jesus Christ was alive, as were the thieves, they were sent to strike them: but when they came, they found our Lord Jesus Christ dead; then one of the soldiers touched his side with a spear, and water and blood flowed from his side (John 19:33 et seq.). Why water? Why blood? Water, to cleanse; blood, to redeem. Why from the side? Because from where there is fault, there is also grace. Fault through the woman, grace through the Lord Jesus Christ (De Consec., dist. 2, c. In calicem, § De latere).