Objection 1. It seems that the body of Christ is not in this sacrament in very truth, but only as in a figure, or sign. For it is written (John 6:54) that when our Lord had uttered these words: "Except you eat the flesh of the Son of Man, and drink His blood," etc., "Many of His disciples on hearing it said: 'this is a hard saying'": to whom He rejoined: "It is the spirit that quickeneth; the flesh profiteth nothing": as if He were to say, according to Augustine's exposition on Ps. 4 [On Psalm 98:9]: "Give a spiritual meaning to what I have said. You are not to eat this body which you see, nor to drink the blood which they who crucify Me are to spill. It is a mystery that I put before you: in its spiritual sense it will quicken you; but the flesh profiteth nothing."
Objection 2. Further, our Lord said (Matthew 28:20): "Behold I am with you all days even to the consummation of the world." Now in explaining this, Augustine makes this observation (Tract. xxx in Joan.): "The Lord is on high until the world be ended; nevertheless the truth of the Lord is here with us; for the body, in which He rose again, must be in one place; but His truth is spread abroad everywhere." Therefore, the body of Christ is not in this sacrament in very truth, but only as in a sign.
Objection 3. Further, no body can be in several places at the one time. For this does not even belong to an angel; since for the same reason it could be everywhere. But Christ's is a true body, and it is in heaven. Consequently, it seems that it is not in very truth in the sacrament of the altar, but only as in a sign.
Objection 4. Further, the Church's sacraments are ordained for the profit of the faithful. But according to Gregory in a certain Homily (xxviii in Evang.), the ruler is rebuked "for demanding Christ's bodily presence." Moreover the apostles were prevented from receiving the Holy Ghost because they were attached to His bodily presence, as Augustine says on John 16:7: "Except I go, the Paraclete will not come to you" (Tract. xciv in Joan.). Therefore Christ is not in the sacrament of the altar according to His bodily presence.
On the contrary, Hilary says (De Trin. viii): "There is no room for doubt regarding the truth of Christ's body and blood; for now by our Lord's own declaring and by our faith His flesh is truly food, and His blood is truly drink." And Ambrose says (De Sacram. vi): "As the Lord Jesus Christ is God's true Son so is it Christ's true flesh which we take, and His true blood which we drink."
I answer that, The presence of Christ's true body and blood in this sacrament cannot be detected by sense, nor understanding, but by faith alone, which rests upon Divine authority. Hence, on Luke 22:19: "This is My body which shall be delivered up for you," Cyril says: "Doubt not whether this be true; but take rather the Saviour's words with faith; for since He is the Truth, He lieth not."
Now this is suitable, first for the perfection of the New Law. For, the sacrifices of the Old Law contained only in figure that true sacrifice of Christ's Passion, according to Hebrews 10:1: "For the law having a shadow of the good things to come, not the very image of the things." And therefore it was necessary that the sacrifice of the New Law instituted by Christ should have something more, namely, that it should contain Christ Himself crucified, not merely in signification or figure, but also in very truth. And therefore this sacrament which contains Christ Himself, as Dionysius says (Eccl. Hier. iii), is perfective of all the other sacraments, in which Christ's virtue is participated.
Secondly, this belongs to Christ's love, out of which for our salvation He assumed a true body of our nature. And because it is the special feature of friendship to live together with friends, as the Philosopher says (Ethic. ix), He promises us His bodily presence as a reward, saying (Matthew 24:28): "Where the body is, there shall the eagles be gathered together." Yet meanwhile in our pilgrimage He does not deprive us of His bodily presence; but unites us with Himself in this sacrament through the truth of His body and blood. Hence (John 6:57) he says: "He that eateth My flesh, and drinketh My blood, abideth in Me, and I in him." Hence this sacrament is the sign of supreme charity, and the uplifter of our hope, from such familiar union of Christ with us.
Thirdly, it belongs to the perfection of faith, which concerns His humanity just as it does His Godhead, according to John 14:1: "You believe in God, believe also in Me." And since faith is of things unseen, as Christ shows us His Godhead invisibly, so also in this sacrament He shows us His flesh in an invisible manner.
Some men accordingly, not paying heed to these things, have contended that Christ's body and blood are not in this sacrament except as in a sign, a thing to be rejected as heretical, since it is contrary to Christ's words. Hence Berengarius, who had been the first deviser of this heresy, was afterwards forced to withdraw his error, and to acknowledge the truth of the faith.
Reply to Objection 1. From this authority the aforesaid heretics have taken occasion to err from evilly understanding Augustine's words. For when Augustine says: "You are not to eat this body which you see," he means not to exclude the truth of Christ's body, but that it was not to be eaten in this species in which it was seen by them. And by the words: "It is a mystery that I put before you; in its spiritual sense it will quicken you," he intends not that the body of Christ is in this sacrament merely according to mystical signification, but "spiritually," that is, invisibly, and by the power of the spirit. Hence (Tract. xxvii), expounding John 6:64: "the flesh profiteth nothing," he says: "Yea, but as they understood it, for they understood that the flesh was to be eaten as it is divided piecemeal in a dead body, or as sold in the shambles, not as it is quickened by the spirit... Let the spirit draw nigh to the flesh... then the flesh profiteth very much: for if the flesh profiteth nothing, the Word had not been made flesh, that It might dwell among us."
Reply to Objection 2. That saying of Augustine and all others like it are to be understood of Christ's body as it is beheld in its proper species; according as our Lord Himself says (Matthew 26:11): "But Me you have not always." Nevertheless He is invisibly under the species of this sacrament, wherever this sacrament is performed.
Reply to Objection 3. Christ's body is not in this sacrament in the same way as a body is in a place, which by its dimensions is commensurate with the place; but in a special manner which is proper to this sacrament. Hence we say that Christ's body is upon many altars, not as in different places, but "sacramentally": and thereby we do not understand that Christ is there only as in a sign, although a sacrament is a kind of sign; but that Christ's body is here after a fashion proper to this sacrament, as stated above.
Reply to Objection 4. This argument holds good of Christ's bodily presence, as He is present after the manner of a body, that is, as it is in its visible appearance, but not as it is spiritually, that is, invisibly, after the manner and by the virtue of the spirit. Hence Augustine (Tract. xxvii in Joan.) says: "If thou hast understood" Christ's words spiritually concerning His flesh, "they are spirit and life to thee; if thou hast understood them carnally, they are also spirit and life, but not to thee."
Source: Summa Theologica (New Advent)