Why Is the Doctrine of the Eucharist Important?

The past three weeks in Mass the gospel readings have been from John 6 and the homilies have focused on the Eucharist. So it is very timely that commenter Anette asked "Regarding the bread and the wine really becoming flesh and blood. Why is this important? Isn't the presence of Christ what matters?"
I understand what you are saying- that His presence is really what matters! It's just that Catholics believe that receiving his body in blood in the Eucharist is the most direct way to experience his presence and avail ourselves of His grace. This is a doctrine that is very near and dear to me because once my eyes were opened to His presence in the breaking of the bread(the way the early Christians referred to the Mass) I knew I had to return to this Church because it had always been my desire to experience him as fully as possible. I still remember the feeling I had once I realized the early Christians actually believed that Jesus was still with them by way of the Eucharistic celebration/sacrifice of the altar. It was one of those "I could of had a V8" moments multiplied exponentially. I have blogged on this extensively but because of my non-existent system of archiving posts, I am going to answer Anette's question here once again using my thoughts and readings from the past 5 years.

The bread and wine becoming Christ's body and blood is very important because it is the normative way that the Christian world for 2000 has experienced the presence of Christ. It does not preclude us from experiencing Him in other ways but it is indeed the source and summit of a Catholic's faith. It is the ultimate sacrament above all other sacraments.(God's use of physical means to convey His grace) Why do we need a sacrament to fully experience God? I suspect it has something to do with the incarnation, God coming to earth as a man. He chose to use flesh and the stuff of earth to redeem us. Blood, water, bread and wine. Tertullian said that the flesh is the hinge of salvation.

The concept of receiving Christ in the Eucharist is pre-figured in the Old Testament. In the NT, John calls Jesus the Lamb of God and hinges the past with the future for the Jewish disciples. When the followers of John looked down the river and heard John say "Behold the Lamb of God" the picture that came to their mind was the Passover and the sacrificed lamb and the sure knowledge that they must eat the Lamb in in order to escape certain death. Only with a Jewish mind do I suspect we could really gather the import of those words spoken by John about his cousin.(Check this post here) Later, we hear Jesus tell us that He is the bread come down from heaven and we must eat his body and blood. Still later, the night he was betrayed, he took the bread and broke it and said: "Take and eat this is my body" Still later, as the early church started to grow, Paul had to speak to the Corinthians about their lack of discernment of the Lord's body and blood and how some were even getting sick and dying because of their abuse of the sacrament.

Please take a look at what the earliest Christians did with these words and the teachings that the disciples handed down to them. These early Christians believed that Christ would become physically and spiritually present to them in the breaking of the bread. They wrote about it extensively and defended this belief to their deaths and continued to celebrate the Mass for the next 1600 years with almost no dissent in this belief. Here's just a brief story example: Ignatius was a disciple of John, Jesus beloved disciple. He wrote about the Eucharist describing it as the body and blood of our Lord and admonished those who refused to accept it. That was just a mere 70 years after Jesus ascended to heaven. It is unlikely they could have twisted this doctrine wrong so soon. Especially being handed to them from one of the twelve original disciples.

There are pages and pages of writings of the early Church fathers that show that the early Church believed in the real presence which you can access, but my point is that this has been a constant teaching of the Church for 2000 years. There was one or two occasions in history(pre-reformation) when the doctrine of the Eucharist was challenged. Most notably was a priest named Berengar of Tours in the 11th century who argued that it was not necessary for the elements to be changed into his real body and blood, but he was the exception and almost universally this belief has been held. Even when the great schism of 1054 occurred, the Orthodox continued to carry with them this apostolic teaching and to this day we believe Christ is present in the Eucharist confected in an Orthodox Church because they can trace the succession of their priests and bishops to the original apostles.
So the early disciples believed it, the early Church believed it and it was a doctrine rarely challenged in the history of the Church. Even Luther*, at least initially, held to this belief and fought vehemently with the other reformers(Zwingli) who wished to state that the Lord's Supper was symbolic.
When you think about it, why would anyone rail against this doctrine? To believe that Jesus can still come to us in the appearance of bread and wine and give us himself, body soul and divinity? To me, it is one of the most wonderful aspects of this incarnate faith we share. Martin Luther said this in his defense of the Real Presence:

Who, but the devil, has granted such license of wresting the words of the holy Scripture? Who ever read in the Scriptures, that my body is the same as the sign of my body? or, that is is the same as it signifies? What language in the world ever spoke so? It is only then the devil, that imposes upon us by these fanatical men. Not one of the Fathers of the Church, though so numerous, ever spoke as the Sacramentarians: not one of them ever said, It is only bread and wine; or, the body and blood of Christ is not there present.

Surely, it is not credible, nor possible, since they often speak, and repeat their sentiments, that they should never (if they thought so) not so much as once, say, or let slip these words: It is bread only; or the body of Christ is not there, especially it being of great importance, that men should not be deceived. Certainly, in so many Fathers, and in so many writings, the negative might at least be found in one of them, had they thought the body and blood of Christ were not really present: but they are all of them unanimous.”

Regarding the Eucharist he also said: "For it is dangerous and dreadful to hear or believe anything against the unanimous testimony, faith, and doctrine of the entire holy Christian Church, as it has been held unanimously in all the world up to this year 1500."

So in conclusion, it is important because the Eucharist is the means in which Christ promised to abide with us, nourish us and ultimately bring us to salvation.

St Ignatius said in the 2nd Century: "Every time this mystery is celebrated, 'the work of our redemption is carried on' and we 'break the one bread that provides the medicine of immortality, the antidote for death, and the food that makes us live forever in Jesus Christ'"

*What is the sacrament of the altar?

It is the true body and blood of our Lord Jesus Christ under bread and wine for us Christians to eat and to drink, established by Christ Himself. ( cf Luther's little Instruction Book.)