We saw the Prince of Priests coming to us, we saw and heard Him offering His blood for us. We follow, inasmuch as we are able, being priests; and we offer the sacrifice on behalf of the people. And even if we are of but little merit, still, in the sacrifice, we are honorable. For even if Christ is not now seen as the one who offers the sacrifice, nevertheless it is He Himself that is offered in sacrifice here on earth when the body of Christ is offered. Indeed, to offer Himself He is made visible in us, he whose word makes holy the sacrifice that is offered. (Commentaries on Psalms 38:25)
A priest must offer something in sacrifice and according to the Law he must enter the holy place through blood. Therefore, because God had repudiated the blood of bulls and of rams, it was necessary for this Priest, as you have read, to enter into the Holy of Holies, penetrating the heights of heaven, by means of His own blood, so that He might become an eternal oblation for our sins. Priest and Victim, therefore, are one and the same. But the priesthood and the sacrifice are a duty of the human condition; for like a lamb He was led to the slaughter, and He is a priest according to the order of Melchisedech. (The Faith 3:11:87)
"My flesh is truly food and My blood is truly drink." You hear Him speak of His flesh, you hear Him speak of His blood, you know the sacred signs of the Lord's death; and do you worry about His divinity? Hear His words when he says: "A spirit has not flesh and bones." As often as we receive the sacramental elements which through the mystery of the sacred prayer are transformed into the flesh and blood of the Lord, we proclaim the death of the Lord. (The Faith 4:10:124)
Perhaps you may be saying: I see something else; how can you assure me that I am receiving the Body of Christ? It but remains for us to prove it. And how many are the examples we might use! Let us prove that this is not what nature has shaped it to be, but what the blessing has consecrated; for the power of the blessing is greater than that of nature, because by the blessing even nature itself is changed...Christ is in that Sacrament, because it is the Body of Christ; yet, it is not on that account corporeal food, but spiritual. Whence also His Apostle says of the type: "For our fathers ate spiritual food and drank spiritual drink" (1 Cor 10:2-4; 15:44). For the body of God is a spiritual body. (The Mysteries 9:50; 9:58)
But if the blessing of man had such power as to change nature, what are we to say of that divine consecration where the very words of the Lord and Saviour operate? For that sacrament which you receive is made what it is by the word of Christ. But if the word of Elijah had such power as to bring down fire from heaven, shall not the word of Christ have power to change the nature of the elements? You read concerning the making of the whole world: "He spoke and they were made, He commanded and they were created." Shall not the word of Christ, which was able to make out of nothing that which was not, be able to change things which already are into what they were not? For it is not less to give a new nature to things than to change them. (The Mysteries, par. 52)
You may perhaps say: "My bread is ordinary." But that bread is bread before the words of the Sacraments; where the consecration has entered in, the bread becomes the flesh of Christ. And let us add this: How can what is bread be the Body of Christ? By the consecration. The consecration takes place by certain words; but whose words? Those of the Lord Jesus. Like all the rest of the things said beforehand, they are said by the priest; praises are referred to God, prayer of petition is offered for the people, for kings, for other persons; but when the time comes for the confection of the venerable Sacrament, then the priest uses not his own words but the words of Christ. Therefore it is the word of Christ that confects this Sacrament....Before it be consecrated it is bread; but where the words of Christ come in, it is the Body of Christ. Finally, hear Him saying: "All of you take and eat of this; for this is My Body." And before the words of Christ the chalice is full of wine and water; but where the words of Christ have been operative it is made the Blood of Christ, which redeems the people. (The Sacraments 4:4:14; 4:5:23)