The Eucharist is the center of the Catholic Faith, but it is unfortunately a source of great misunderstanding and unbelief. There are people who do not believe that the Eucharist is truly the body and blood of Christ, but that it is merely a symbol of Jesus or that the body and blood of Jesus remain in conjunction with the bread and wine. The Church teaches that the substance of the bread and wine is fully changed into the body and blood of Christ, so that although the appearances of bread and wine remain, it is indeed the body and blood of Christ which are present.
In the Bible – Really the Body and Blood
Jesus speaks of the need to eat His body and drink His blood in great detail in John 6. That chapter begins with the multiplication of the loaves and fishes, in which Jesus feeds the bodily needs of the crowd. He then uses this occasion to speak of eternal life and feeding one's soul. He speaks in very specific terms, telling the crowd that in order to have eternal life, they must eat His body and drink His blood. What is astonishing, though, is that many of His disciples did not like this teaching and decided to stop following Jesus. He let them leave, instead of trying to bring them back by telling them that He only meant His teaching in a figurative sense. Instead, He asked His apostles if they also wanted to leave. This teaching was important enough that He was willing to lose all followers rather than modify it to become more acceptable.
St. Paul also speaks at length about His understanding of the Eucharist. “For I received from the Lord what I also delivered to you, that the Lord Jesus on the night when he was betrayed took bread, and when he had given thanks, he broke it, and said, 'This is my body which is for you. Do this in remembrance of me.' In the same way also the cup, after supper, saying, 'This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me.' For as often as you eat this bread and drink the cup, you proclaim the Lord's death until he comes. Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of profaning the body and blood of the Lord. Let a man examine himself, and so eat of the bread and drink of the cup. For any one who eats and drinks without discerning the body eats and drinks judgment upon himself.” - 1 Corinthians 11: 23 – 29. Paul recounts the institution of the Eucharist from the Gospels, and then he speaks of profaning the body and blood of Christ. Clearly he sees the bread and wine, after their transformation, as indeed the body of blood of Christ, for he says that unworthily eating them brings judgment.
Early Church Fathers – Really the Body and Blood
Since this is such an important topic in the life of the Church, many early Christian writers had plenty to say about it, and they all re-affirmed that the bread and wine are indeed changed into the body and blood of Christ.
For not as common bread nor common drink do we receive these; but since Jesus Christ our Savior was made incarnate by the word of God and had both flesh and blood for our salvation, so too, as we have been taught, the food which has been made into the Eucharist by the Eucharistic prayer set down by Him, and by the change of which our blood and flesh is nourished, is both the flesh and the blood of that incarnated Jesus. - Justin Martyr First Apology, 66
We give thanks to the Creator of all, and, along with thanksgiving and prayer for the blessings we have received, we also eat the bread presented to us; and this bread, becomes by prayer a sacred body, which sanctifies those who sincerely partake of it. - Origen Against Celsus 8:33
Then having sanctified ourselves by these spiritual Hymns, we beseech the merciful God to send forth His Holy Spirit upon the gifts lying before Him; that He may make the Bread the Body of Christ, and the Wine the Blood of Christ; for whatsoever the Holy Ghost has touched, is surely sanctified and changed. - Cyril of Jerusalem Catechetical Lectures 23.7
They [i.e. the Gnostics] abstain from the Eucharist and from prayer, because they do not confess that the eucharist is the flesh of our savior Jesus Christ, flesh which suffered for our sins and which the Father, in his goodness, raised up again. - Ignatius of Antioch Letter to the Smyrnians 7:1
You shall see the Levites bringing loaves and a cup of wine, and placing them on the table. So long as the prayers of supplication and entreaties have not been made, there is only bread and wine. But after the great and wonderful prayers have been completed, then the bread is become the Body, and the wine the Blood, of our Lord Jesus Christ…Let us approach the celebration of the mysteries. This bread and this wine, so long as the prayers and supplications have not taken place, remain simply what they are. But after the great prayers and the holy supplications have been sent forth, the Word comes down into the bread and wine – and thus is His body confected. - Athanasius Sermon to the Newly Baptized
To communicate each day and to partake of the holy Body and Blood of Christ is good and beneficial; for He says quite plainly: “He that eats My Flesh and drinks My Blood has eternal life.” Who can doubt that to share continually in life is the same thing as having life abundantly? We ourselves communicate four times each week…and on other days if there is a commemoration of any saint. - Basil the Great Letter of Basil to a Patrician Lady Caesaria
The bread again is at first common bread; but when the mystery sanctifies it, it is called and actually becomes the Body of Christ. So too the mystical oil, so too the wine; if they are things of little worth before the blessing, after their sanctification by the Spirit each of them has its own superior operation. This same power of the word also makes the priest venerable and honorable, separated from the generality of men by the new blessing bestowed upon him. - Gregory of Nyssa Sermon on the Day of Lights or On the Baptism of Christ
It is not man that causes the things offered to become the Body and Blood of Christ, but he who was crucified for us, Christ himself. The priest, in the role of Christ, pronounces these words, but their power and grace are God’s. ‘This is my body,’ he says. This word transforms the things offered. – John Chrysostom Against the Judaizers 1.6
Christ said indicating (the bread and wine): ‘This is My Body,’ and ‘This is My Blood,’ in order that you might not judge what you see to be a mere figure. The offerings, by the hidden power of God Almighty, are changed into Christ's Body and Blood, and by receiving these we come to share in the life-giving and sanctifying efficacy of Christ. – Cyril of Alexandria Commentary on Matthew 26, 27
All these writers speak of a transformation of the bread and wine into the Body and Blood of Christ when the words of consecration are pronounced. They are very clear that the bread and wine become something different, namely the Body and Blood of Christ.
In the Bible Unworthy Communion
Receiving Jesus in Holy Communion is a very great gift, and one for which we should always be thankful. We are never worthy to receive Holy Communion because we are sinful and God is completely pure. However, God gives Himself to us in Communion to strengthen. However, there are times when we should refrain from receiving Communion. “Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be answerable for the body and blood of the Lord. Examine yourselves, and only then eat of the bread and drink of the cup. For all who eat and drink without discerning the body, eat and drink judgment against themselves." - 1 Corinthians 11: 27 – 29. St. Paul warns us not to partake of the Body and Blood of Christ if we have sinned seriously. This makes sense, because serious sin is a complete rejection of God. It would be presumptuous to partake of Christ's body and blood while completely rejecting Him. This is what St. Paul is warning against.