I am the bread of life. Your ancestors ate the manna in the desert, but they died; this is the bread that comes down from heaven so that one may eat it and not die. I am the living bread that came down from Heaven; whoever eats this bread will live forever; and the bread that I will give is my flesh for the life of the world. (Jn 6:48-51)

Jesus said to them, ‘Amen, amen, I say to you, unless you eat the flesh of the Son of Man and drink his blood, you do not have life within you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him on the last day. For my flesh is true food, and my blood is true drink.’ (Jn 6:53-55)

While they were eating, Jesus took bread, said the blessing, broke it, and giving it to his disciples said, ‘Take and eat; this is my body.’ Then he took a cup, gave thanks, and gave it to them, saying, ‘Drink from it, all of you, for this is my blood of the covenant, which will be shed on behalf of many for the forgiveness of sins.’ (Mt 26:26-28)

This [the Eucharist] is a true Sacrament, and one of those seven which the holy Church has ever revered and venerated religiously. For when the consecration of the chalice is effected, it is called a mystery of faith. (Catechism of Trent, The Sacrament of the Eucharist, no. 2300)

The Eucharist is a Sacrament in which, by the marvelous conversion of the whole substance of bread into the Body of Jesus Christ, and that of wine into His precious Blood, is contained truly, really, and substantially, the Body, the Blood, the Soul and Divinity of the same Lord Jesus Christ, under the appearance of bread and wine as our spiritual food. (Catechism of Saint Pius X, ch. IV, no. 598)

The Eucharist is the very sacrifice of the Body and Blood of the Lord Jesus which he instituted to perpetuate the sacrifice of the cross throughout the ages until his return in glory. Thus he entrusted to his Church this memorial of his death and Resurrection. It is a sign of unity, a bond of charity, a paschal banquet, in which Christ is consumed, the mind is filled with grace, and a pledge of future glory is given to us. (Compendium of the Catechism of the Catholic Church, no. 271)

This sacrament was appropriately instituted at the supper, when Christ conversed with His disciples for the last time. First of all, because of what is contained in the sacrament: for Christ is Himself contained in the Eucharist sacramentally. […]  Secondly, because without faith in the Passion there could never be any salvation, according to Romans 3:25: ‘Whom God hath proposed to be a propitiation, through faith in His blood.’ It was necessary accordingly that there should be at all times among men something to show forth our Lord’s Passion; the chief sacrament of which in the old Law was the Paschal Lamb. Hence the Apostle says (1Cor 5:7): ‘Christ our Pasch is sacrificed.’ But its successor under the New Testament is the sacrament of the Eucharist, which is a remembrance of the Passion now past, just as the other was figurative of the Passion to come. […] Consequently, since, as Pope Alexander I says, ‘among sacrifices there can be none greater than the Body and Blood of Christ, nor any more powerful oblation’; our Lord instituted this sacrament at His last parting with His disciples. (Saint Thomas Aquinas. Summa Theologica, III, q. 73, a.5)

This belief has always been in the Church of God, that immediately after the consecration the true body of our Lord and His true blood together with His soul and divinity exist under the species of bread and wine; […] Therefore, it is very true that as much is contained under either species as under both. For Christ whole and entire exists under the species of bread and under any part whatsoever of that species, likewise the whole (Christ) is present under the species of wine and under its parts. (Denzinger-Hünermann 1640, Council of Trent, Session XIII, Decree on the Most Holy Eucharist, October 11, 1551)

Yet ‘mysterium fidei’ is mentioned, since something is believed there other than what is perceived; and something is perceived other than is believed. For the species of bread and wine is perceived there, and the truth of the body and blood of Christ is believed and the power of unity and of love.  We must, however, distinguish accurately between three things which are different in this sacrament, namely, the visible form, the truth of the body, and the spiritual power. The form is of the bread and wine; the truth, of the flesh and blood; the power, of unity and of charity. (Denzinger-Hünermann 782 –783, Letter Cum Marthae circa to John, Archbishop of Lyons, November 29, 1202)

So Christ our Lord – who, in His Pasion, offered for us that which, in being born, He had received from us – becoming forever the most high priest, ordered that the sacrifice which you see be offered, that is, of His body and his blood. In fact, his body, pierced by the lance, poured forth water and blood, by which he forgave our sins. Reembering this grace, working out your salvation (which is God who is at work in you), with fear and trembling come up to partake of this altar. Recognize in the bread that which hung on the cross, and in the chalice that which gushed out from his side. (Saint Augustine of Hippo. Sermon 228 B: About the Sacraments on Easter Day, no.2)

Even if reason was incapable of understanding, and the word unable to express the reality, it is necessary to consider as true that which since antiquity was believed and preached as the true Catholic faith in the entire Church. (Saint Augustine of Hippo. Contra Iulianum, book 6, ch. 5, no. 11)

The mystery of faith! With these words, spoken immediately after the words of consecration, the priest proclaims the mystery being celebrated and expresses his wonder before the substantial change of bread and wine into the body and blood of the Lord Jesus, a reality which surpasses all human understanding. The Eucharist is a ‘mystery of faith’ par excellence: ‘the sum and summary of our faith’ (Catechism of the Catholic Church, 1327) The Church’s faith is essentially a Eucharistic faith, and it is especially nourished at the table of the Eucharist. […] For this reason, the Sacrament of the Altar is always at the heart of the Church’s life: ‘thanks to the Eucharist, the Church is reborn ever anew!’ (Benedict XVI, Homily at the Mass of Installation in the Cathedral of Rome – 7 May 2005): AAS 97 (2005): 752). The more lively the eucharistic faith of the People of God, the deeper is its sharing in ecclesial life in steadfast commitment to the mission entrusted by Christ to his disciples. (Benedict XVI. Apostolic Exhortation Sacramentum caritatis, no. 6, February 22, 2007)

Hilary says (De Trin., 8): ‘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., 6): ‘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.’
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.’ (Saint Thomas Aquinas. Summa Theologica III, q. 75, a.1)

Whoever eats my flesh and drinks my blood remains in me and I in him.  Just as the living Father sent me and I have life because of the Father, so also the one who feeds on me will have life because of me. (Jn 6:56-57)

In the sacrament of the altar, the Lord meets us, men and women created in God’s image and likeness (cf. Gen 1:27), and becomes our companion along the way. In this sacrament, the Lord truly becomes food for us, to satisfy our hunger for truth and freedom. Since only the truth can make us free (cf. Jn 8:32), Christ becomes for us the food of truth. (Benedict XVI. Apostolic Exhortation Sacramentum caritatis, no. 2, February 22, 2007)

Holy Communion augments our union with Christ. The principal fruit of receiving the Eucharist in Holy Communion is an intimate union with Christ Jesus. Indeed, the Lord said: ‘He who eats my flesh and drinks my blood abides in me, and I in him’ (Jn 6:56). Life in Christ has its foundation in the Eucharistic banquet: ‘As the living Father sent me, and I live because of the Father, so he who eats me will live because of me’ (Jn 6:57). (Catechism of the Catholic Church, no. 1391)

That which when angels behold, they tremble, and dare not so much as look up at it without awe on account of the brightness that cometh thence, with this we are fed with this we are commingled, and we are made one body and one flesh with Christ. […] Himself feeds us with His own blood, and by all means entwines us with Himself. (Saint John Chrysostom. Homilies on the Gospel of Saint Matthew, hom. 82, no. 5)

But although there are two elements, as bread and wine, of which the entire Sacrament of the Eucharist is constituted, yet guided by the authority of the Church, we confess that this is not many Sacraments, but only one. Otherwise, there cannot be the exact number of seven Sacraments, as has ever been handed down. […] Moreover, by virtue of the Sacrament, one mystical body is effected; hence, that the Sacrament itself may correspond to the thing which it effects, it must be one. (Catechism of Trent, no. 2300, The Sacrament of the Eucharist)

Sharing in the Eucharist, the sacrament of the New Covenant (cf. 1 Cor 11:23-29), is the culmination of our assimilation to Christ. (John Paul II. Encyclical Veritatis splendor, no. 21 August 6, 1993)

‘We have become Christ. For if he is the head we are the members; he and we together are the whole man’: St Augustine’s bold words (Tract. in Joh., 21, 8) extol the intimate communion that is created between God and man in the mystery of the Church, a communion which, on our journey through history, finds its supreme sign in the Eucharist. The commands, ‘Take, eat … Drink of it …’ (Mt 26:26-27), which Jesus gives his disciples in that room on the upper floor of a house in Jerusalem on the last evening of his earthly life (cf. Mk 14:15), are rich in meaning. The universal symbolic value of the banquet offered in bread and wine (cf. Is 25:6) already suggests communion and intimacy. Other more explicit elements extol the Eucharist as a banquet of friendship and covenant with God. (John Paul II. General audience, no. 1, October 18, 2000)

In this sacrament, as in the others, that which is a sacrament is a sign of the reality of the sacrament. Now there is a twofold reality of this sacrament, as stated above (Question 73, Article 6): one which is signified and contained, namely, Christ Himself; while the other is signified but not contained, namely, Christ’s mystical body, which is the fellowship of the saints. Therefore, whoever receives this sacrament, expresses thereby that he is made one with Christ, and incorporated in His members. (Saint Thomas Aquinas. Summa Theologica III, q. 80, a. 4)

This sacrament has a threefold significance. one with regard to the past, inasmuch as it is commemorative of our Lord’s Passion, which was a true sacrifice, as stated above (q. 48, a. 3), and in this respect it is called a ‘Sacrifice.’ With regard to the present it has another meaning, namely, that of Ecclesiastical unity, in which men are aggregated through this Sacrament; and in this respect it is called Communio or Synaxis. For Damascene says (De Fide Orth., 4) that ‘it is called Communion because we communicate with Christ through it, both because we partake of His flesh and Godhead, and because we communicate with and are united to one another through it.’ (Saint Thomas Aquinas. Summa Theologica III, q. 73, a. 4)

The Church is a ‘communion of saints’: this expression refers first to the ‘holy things’ (sancta), above all the Eucharist, by which ‘the unity of believers, who form one body in Christ, is both represented and brought about’ (LG 3). (Catechism of the Catholic Church, no. 960)

This is why Christian antiquity used the same words, Corpus Christi, to designate Christ’s body born of the Virgin Mary, his Eucharistic body and his ecclesial body. This clear datum of the tradition helps us to appreciate the inseparability of Christ and the Church. The Lord Jesus, by offering himself in sacrifice for us, in his gift effectively pointed to the mystery of the Church. It is significant that the Second Eucharistic Prayer, invoking the Paraclete, formulates its prayer for the unity of the Church as follows: ‘may all of us who share in the body and blood of Christ be brought together in unity by the Holy Spirit.’ These words help us to see clearly how the res of the sacrament of the Eucharist is the unity of the faithful within ecclesial communion. The Eucharist is thus found at the root of the Church as a mystery of communion (cf. STh, III, 80, 4). (Benedict XVI. Apostolic Exhortation Sacramentum caritatis, no. 15, February 22, 2007)

The Eucharist, in consequence, is our daily bread. But we must receive it as refreshment for the body, but as sustenance for the spirit. The effect that is fitting to this nutrition is to produce unity, in order that, integrated into the body of Christ, constituted as his members, we become that which we receive. And so it will effectively be our daily bread. (Saint Augustine. Sermon 57, On the Lord’s Prayer, no. 7)

When we say ‘give us this day our daily bread,’ we can understand this as perfectly referring also to the Eucharist, the daily nourishment. In effect, the faithful know what they receive and that for them it is a good to receive this daily bread, that is necessary in this life. They pray for themselves in order to become better and to persevere in goodness, in faith and in an upright life. This they desire, this they beseech in prayer for, if they do not persevere in a good life, they will be dispossessed of that bread. Hence, what is the meaning of: ‘give us this day our daily bread?’ May we live in such a way as to not be separated from your altar.  (Saint Augustine. Sermon 58, On the Lord`s Prayer no. 5)

Everyone knows that John himself, the Apostle of love, who seems to reveal in his Gospel the secrets of the Sacred Heart of Jesus, and who never ceased to impress on the memories of his followers the new commandment ‘Love one another,’ altogether forbade any intercourse with those who professed a mutilated and corrupt version of Christ’s teaching: ‘If any man come to you and bring not this doctrine, receive him not into the house nor say to him: God speed you’ (2 John 10). (Pius XI. Encyclical Mortalium animos, no. 9, January 5, 1928)

Scattered and separated members cannot possibly cohere with the head so as to make one body. But Saint Paul says: ‘All members of the body, whereas they are many, yet are one body, so also is Christ’ (1 Cor. 12:12). Wherefore this mystical body, he declares, is ‘compacted and fitly jointed together. The head, Christ: from whom the whole body, being compacted and fitly jointed together, by what every joint supplieth according to the operation in the measure of every part’ (Eph.4:15-16). And so dispersed members, separated one from the other, cannot be united with one and the same head. (Leo XIII. Encyclical Satis cognitum, no. 9, June 26, 1896)

The Church, founded on these principles and mindful of her office, has done nothing with greater zeal and endeavour than she has displayed in guarding the integrity of the faith. Hence she regarded as rebels and expelled from the ranks of her children all who held beliefs on any point of doctrine different from her own. (Leo XIII. Encyclical Satis cognitum, no. 9, June 26, 1896)

Ever since the origins of the early Church, due to the necessity that the faith of the elect be proven like gold in the fire, the Apostle, vessel of election, wished to warn the faithful that if someone arise, of those who alter and amend the Gospel of Christ, spreading false doctrines and betraying the depository of the faith, even if it was an angel who evangelized something other than he had, it would be necessary to anathemize him. (Pius IX. Apostolic Letter Ad Apostolicae Sedis, August 22, 1851)

It is owing to the Savior’s infinite mercy that place is allowed in His Mystical Body here below for those whom, of old, He did not exclude from the banquet (cf. Mt, 9:11; Mk 2:16; Lk, 15:2). For not every sin, however grave it may be, is such as of its own nature to sever a man from the Body of the Church, as does schism or heresy or apostasy. (Pius XII. Encyclical Mystici Corporis, no. 23, June 29, 1943)

Receive, therefore, and eat the body of Christ; now that you too have become members of Christ in the body of Christ; receive and drink the blood of Christ. So as not to be separated, eat what unites you; in order not to seem cheap in your own estimation, drink the price paid for you. […] So then, if you have life in him, you will be with him in one flesh. This sacrament, after all, does not give you the body of Christ so as then to divide you in it. […] You, then, begin to receive what you have already begun to be, provided that you do not receive it unworthily, eating and drinking your own condemnation. […] And you receive it worthily, if you keep far from the yeast of bad doctrines, so that you be ‘unleavened loaves of sincerity and truth’ (1 Cor 5:8). (Saint Augustine of Hippo. Sermon 228 B, About the Sacraments, on Easter Day, nos. 3-5)

This is what the universal Church does today, for which is prepared today by Christ on Mount Sion a splendid and sumptuous banquet, that is, with a double richness: interior and exterior, and abundant. He gave his true body, rich in all spiritual power and filled with interior and exterior charity; and commanded it to be given to that those who believe in Him. That is why one must firmly believe and orally confess that the body, which the Virgin bore, which hung on the cross, lay in the sepulcher, resurrected on the third day and rose into heaven to the right hand of the Father, He, today, really gave it to his apostles; and the Church, everyday, really consecrates and distributes it to the faithful. (Saint Anthony of Padua. Sermo de Cena Domini, Sermo Allegoricus II, no. 6 – Spanish)

We must now return to an explanation of those truths concerning the Eucharist about which the faithful are on no account to be left in ignorance. Pastors, aware of the warning of the Apostle that those who discern not the body of the Lord are guilty of a most grave crime, should first of all impress on the minds of the faithful the necessity of detaching, as much as possible, their mind and understanding from the dominion of the senses; for if they believe that this Sacrament contains only what the senses disclose, they will of necessity fall into enormous impiety. Consulting the sight, the touch, the smell, the taste and finding nothing but the appearances of bread and wine, they will naturally judge that this Sacrament contains nothing more than bread and wine. Their minds, therefore, are as much as possible to be withdrawn from subjection to the senses and excited to the contemplation of the stupendous might and power of God. The Catholic Church firmly believes and professes that in this Sacrament the words of consecration accomplish three wondrous and admirable effects.The first is that the true body of Christ the Lord, the same that was born of the Virgin, and is now seated at the right hand of the Father in heaven, is contained in this Sacrament. The second, however repugnant it may appear to the senses, is that none of the substance of the elements remains in the Sacrament. The third, which may be deduced from the two preceding, although the words of consecration themselves clearly express it, is that the accidents which present themselves to the eyes or other senses exist in a wonderful and ineffable manner without a subject. All the accidents of bread and wine we can see, but they inhere in no substance, and exist independently of any; for the substance of the bread and wine is so changed into the body and blood of our Lord that they altogether cease to be the substance of bread and wine. (Catechism of Trent, no. 2300, The Sacrament of the Eucharist)

First of all the holy Synod teaches and openly and simply professes that in the nourishing sacrament of the Holy Eucharist after the consecration of the bread and wine our Lord Jesus Christ, true God and man, is truly, really, and substantially [can. I] contained under the species of those sensible things. […] it is a most disgraceful thing for some contentious and wicked men to distort into fictitious and imaginary figures of speech, by which the real nature of the flesh and blood of Christ is denied, contrary to the universal sense of the Church, which, recognizing with an ever grateful and recollecting mind this most excellent benefit of Christ, as the pillar and ground of truth (1Tim 3:15), has detested these falsehoods, devised by impious men, as satanical. (Denzinger-Hünermann 1636, 1637. Council of Trent, Session XIII, October 11, 1551)

Now he is in bondage to a sign who uses, or pays homage to, any significant object without knowing what it signifies:  he, on the other hand, who either uses or honors a useful sign divinely appointed, whose force and significance he understands, does not honor the sign which is seen and temporal, but that to which all such signs refer. […] But at the present time, after that the proof of our liberty has shone forth so clearly in the resurrection of our Lord, we are not oppressed with the heavy burden of attending even to those signs which we now understand, but our Lord Himself, and apostolic practice, have handed down to us a few rites in place of many, and these at once very easy to perform, most majestic in their significance, and most sacred in the observance; such, for example, as the sacrament of Baptism, and the celebration of the Body and Blood of the Lord.  And as soon as any one looks upon these observances he knows to what they refer, and so reveres them not in carnal bondage, but in spiritual freedom.  Now, as to follow the letter, and to take signs for the things that are signified by them, is a mark of weakness and bondage; so to interpret signs wrongly is the result of being misled by error. […] And it is better even to be in bondage to unknown but useful signs than, by interpreting them wrongly, to draw the neck from under the yoke of bondage only to insert it in the coils of error. (Saint Augustine of Hippo. On Christian Doctrine, Book III, ch. 9, no. 13)

He wished, furthermore, that this be a pledge of our future glory and of everlasting happiness, and thus be a symbol of that one ‘body’ of which He Himself is the ‘head’ (1 Cor 11:23 Eph 5:23), and to which He wished us to be united, as members, by the closest bond of faith, hope, and charity, that we might ‘all speak the same thing and there might be no schisms among us’ (cf. 1 Cor 1:10). (Denzinger-Hunermann 1638. Council of Trent, Session XIII, Decree on the Most Holy Eucharist, October 11, 1551)

If anyone denies that in the sacrament of the most holy Eucharist there are truly, really, and substantially contained the body and blood together with the soul and divinity of our Lord Jesus Christ, and therefore the whole Christ, but shall say that He is in it as by a sign or figure, or force, let him be anatema. […] If anyone moreover teaches the contrary or preaches or obstinately asserts, or even publicly by disputation shall presume to defend the contrary, by that fact itself he is excommunicated. (Denzinger-Hunermann 1651, Council of Trent, Session XIII, Decree on the Sacrament of the Eucharist, October 11, 1551)

The sacred and holy ecumenical and general Synod of Trent […] namely to publish the true and ancient doctrine concerning faith and the sacraments, and to provide a remedy for all the heresies and other very serious troubles by which the Church of God is at present wretchedly agitated and torn into many different factions, yet from the beginning has had this especially among its desires, to uproot the ‘cockles’ of execrable errors and schisms, which the enemy in these troubled times of our has ‘sown’ (Mt 13:25 ff.), in the doctrine of the faith, in the use and worship of the sacred Eucharist, which our Savior, moreover, left in His Church as a symbol of that unity and charity with which He wished all Christians to be mutually bound and united. Therefore, this same sacred and holy synod, transmitting that sound and genuine doctrine of this venerable and divine sacrament of the Eucharist, which the Catholic Church, instructed by our Lord Jesus Christ himself and by his Apostles, and taught by the ‘Holy Spirit who day by day brings to her all truth’ (Jn 14:26), has always held and will preserve even to the end of time, forbids all the faithful of Christ hereafter to venture to believe, teach, or preach concerning the Most Holy Eucharist otherwise than is explained and defined in this present decree. (Denzinger-Hunermann 1635, Council of Trent, Session XIII, Decree on the Sacrament of the Eucharist, October 11, 1551)

Jesus said to them, ‘I am the bread of life; whoever comes to me will never hunger, and whoever believes in me will never thirst. But I told you that although you have seen (me), you do not believe. (Jn 6:35-36)

Then many of his disciples who were listening said, ‘This saying is hard; who can accept it?’ Since Jesus knew that his disciples were murmuring about this, he said to them, ‘Does this shock you? What if you were to see the Son of Man ascending to where he was before? It is the spirit that gives life, while the flesh is of no avail. The words I have spoken to you are spirit and life. But there are some of you who do not believe.’ (Jn 6:60-64)

Whoever teaches something different and does not agree with the sound words of our Lord Jesus Christ and the religious teaching is conceited, understanding nothing, and has a morbid disposition for arguments and verbal disputes. From these come envy, rivalry, insults, evil suspicions, and mutual friction among people with corrupted minds, who are deprived of the truth, supposing religion to be a means of gain. (1Tim 6:3-5)