from CatholicsComeHome.org

What is the Eucharist?

The Eucharist is the source and summit of the Christian life, because it is Jesus wholly present: body, blood, soul and divinity. 

The Eucharist is also a sacrifice because it makes Jesus’ unique sacrifice on the cross present to us.  At every Mass Jesus unites our sufferings, prayer, and work with the total gift of himself to the Father. 

In Holy Communion we take Jesus into our bodies under the appearance of bread and wine, but he transforms us into himself. The whole Christ is really, truly, and substantially present in the Holy Eucharist. We use the words "really, truly, and substantially" to describe Christ's presence in the Holy Eucharist in order to distinguish Our Lord's teaching from that of mere men who falsely teach that the Holy Eucharist is only a sign or figure of Christ, or that He is present only by His power.
 

Why does the Catholic Church believe Christ is really present in the Eucharist?

The Catholic doctrine of the Real Presence is the belief that Jesus Christ is literally, not symbolically, present in the Holy Eucharist—body, blood, soul and divinity. Catholics believe in the Real Presence of Christ in the Eucharist because Jesus tells us this is true in the Bible:“I am the bread of life. Your fathers ate the manna in the wilderness, and they died. This is the bread which comes down from heaven, that a man may eat of it and not die. I am the living bread which came down from heaven; if any one eats of this bread, he will live for ever; and the bread which I shall give for the life of the world is my flesh. The Jews then disputed among themselves, saying, ‘How can this man give us his flesh to eat?’ So Jesus said to them, “Truly, truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you; he who eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day. For my flesh is food indeed, and my blood is drink indeed. He who eats my flesh and drinks my blood abides in me, and I in him” (John 6:48-56).Furthermore, the early Church Fathers either imply or directly state that the bread and wine offered in the celebration of the Lord’s Supper is really the body and blood of Jesus Christ. In other words, the doctrine of the Real Presence that Catholics believe today was believed by the earliest Christians 2,000 years ago!

This miracle of God’s physical presence to us at every Mass is the truest testament to Christ’s love for us and His desire for each of us to have a personal relationship with Him.

More Scripture about the Real Presence of Christ in the Eucharist:

• (John 6:53-56 RSV) So Jesus said to them, “Truly, truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you; {54} he who eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day. {55} For my flesh is food indeed, and my blood is drink indeed. {56} He who eats my flesh and drinks my blood abides in me, and I in him.

• In the Aramaic language that Our Lord spoke, to symbolically “eat the flesh” or “drink the blood” of someone meant to persecute or assault them. See the following… (Psa 27:2 KJV) When the wicked, even mine enemies and my foes, came upon me to eat up my flesh, they stumbled and fell.

• (Isa 9:18-20 RSV) For wickedness burns like a fire, it consumes briers and thorns; it kindles the thickets of the forest, and they roll upward in a column of smoke. {19} Through the wrath of the LORD of hosts the land is burned, and the people are like fuel for the fire; no man spares his brother. {20} They snatch on the right, but are still hungry, and they devour on the left, but are not satisfied; each devours his neighbor’s flesh,

• (Isa 49:26 RSV) I will make your oppressors eat their own flesh, and they shall be drunk with their own blood as with wine. Then all flesh shall know that I am the LORD your Savior, and your Redeemer, the Mighty One of Jacob.”

• (Micah 3:3 RSV) who eat the flesh of my people, and flay their skin from off them, and break their bones in pieces, and chop them up like meat in a kettle, like flesh in a caldron.

• (2 Sam 23:17 RSV) “Far be it from me, O LORD, that I should do this. Shall I drink the blood of the men who went at the risk of their lives?” Therefore he would not drink it. These things did the three mighty men.

• (Rev 17:6 RSV) And I saw the woman, drunk with the blood of the saints and the blood of the martyrs of Jesus. When I saw her I marveled greatly.

• (Rev 17:16 NIV) The beast and the ten horns you saw will hate the prostitute. They will bring her to ruin and leave her naked; they will eat her flesh and burn her with fire.

Thus, if Jesus were only speaking symbolically about eating His flesh and drinking His blood, as the Protestants say, then what He really meant was “whoever persecutes and assaults me will have eternal life” — which, of course, makes nonsense of the passage!

Bread and wine are not normal or natural symbols of flesh and blood. To call a man a “fox” is an understandable symbol for cleverness. To call a man “bread” is not an understandable symbol, without some explanation. Either the symbols would have been clearly explained (which is not the case) or Jesus spoke literally (which is the case!).

Taken from Beginning Apologetics, How to Explain and Defend the Catholic Faith, by San Juan Catholic Seminars, P.O. Box 5253, Farmington, NM 87499-52539889
 

Why can’t non-Catholics receive Holy Communion in the Catholic Church?

Catholics believe in the Real Presence of Jesus in the Eucharist, meaning that what appears to be bread and wine is really Jesus’ body and blood—not just a symbol of his body and blood. When Catholics receive Holy Communion, it is an expression of the unity among all those in communion with the Catholic Church throughout the world, who maintain the belief in the Real Eucharistic Presence of Christ. Therefore, only those who believe in the True Presence may participate in this sacrament of oneness with Christ and his Church. “… [T]he celebration of the Eucharistic sacrifice is wholly directed toward the intimate union of the faithful with Christ through communion” (CCC 1382).

Ultimately, Catholics believe that we cannot celebrate this unifying sacrament with other Christians while there are disagreements about the Eucharist itself. However, Catholics pray for the day when we can reconcile with other Christians and share in the unity of God’s people through the Holy Eucharist.

The United States Conference of Catholic Bishops expresses this desire for unity:

“We pray that our common baptism and the action of the Holy Spirit in this Eucharist will draw us closer to one another and begin to dispel the sad divisions which separate us. We pray that these will lessen and finally disappear, in keeping with Christ’s prayer for us ‘that they may all be one’” (John 17:21).