Our Divine Lord fulfilled His promise to give His flesh to eat and His blood to drink twelve months later, when the Jews were again celebrating the Paschal Feast, and when He Himself celebrated it for the last time on earth with His disciples. It was Thursday evening, now known as Holy Thursday. The very next day, Good Friday, He was to die on the Cross. Matthew, Mark and Luke, as well as St. Paul in his First Epistle to the Corinthians, all relate the momentous events of that Last Paschal Feast - The Last Supper. These are the words  written by St. Paul to the early Christians of Corinth.

He begins by letting them know that what Jesus had told and what he was now telling them, he had received from Christ Himself: "For I have received of the Lord that which also I delivered unto you, that the Lord Jesus, the same night on which He was betrayed, took bread and giving thanks broke and said: Take you and eat, this is my body, which shall be delivered for you. This do for the commemoration of me. In like manner also the chalice after He had supped, saying: This chalice is the new testament in my blood. This do you as often as you shall drink for the commemoration of me. For as often as you shall eat this bread and drink the chalice, you shall show the death of the Lord until He comes. Therefore, whosoever shall eat this bread, or drink the chalice of the Lord unworthily, (in mortal sin) shall be guilty of the body and the blood of the Lord. But let a man prove himself and so let him eat of that bread and drink of the chalice. For he that eateth and drinketh unworthily, eateth and drinketh judgment to himself, not discerning the body of the Lord." (1Cor. 11: 23-29.)

The Catholic Church has always taught that when Jesus said, "This is my body - This is my blood," He actually changed the substance of the bread and wine into the substance of His body and blood. No longer were bread and wine present, but the real body and blood of Christ under the appearerances  of bread and wine. The Apostles did not receive dead flesh and blood but the whole Christ - His body, blood, soul and divinity. This change is called Transubstantiation. This is a mystery but it is not beyond the power and the love of God to accomplish it. This has always been the teaching of the Church from the earliest ages of Christianity.

St. Paul tells those who receive Holy Communion unworthily, that they are "guilty of the body and of the blood of the Lord," and that they "eat and drink judgment to themselves"  if they do not understand clearly that they are receiving the body of the Lord. Such strong words of condemnation could not be used by him, if the Eucharist were only a symbol. His words signify that they would be guilty of high treason - a crime against the body and blood of the Lord, which in reality they were daring to receive in a state of grave sin. The Fathers of the Church, without exception, use similar language. Cardinal Gibbons counts the names of sixty-three Fathers and eminent writers who lived between the first and the sixth century, all of whom proclaim the Real Presence of Jesus in the Eucharist. (Cf. :Faith of Our Fathers" chapter 21.) St. Ignatius (died A.D. 127) who was a disciple of Sts. Peter and John writes of a sect, the Gnostic's: "They abstain from the Eucharist and prayer because they do not admit that the Eucharist and prayer is the flesh of our Savior Jesus Christ."

If  Protestantism is right, then for fifteen hundred years the Church was sunken in error and idolatry, and Christ's promise failed from the beginning that: "The gates of hell would not prevail against it." But to say this is blasphemy! The History of the Church shows that this doctrine of the Real Presence of Jesus Christ under the appearances of bread and wine was received and believed by the whole Church for fifteen hundred years before the rise of Protestantism.
"Do this for the commemoration of Me." The Catechism tells us that by these words: 1. Jesus ordained His Apostles priests. 2. He gave them power to ordain priests. 3. He commanded all priests in like manner to consecrate, offer and administer His body and His blood.

Source: Rev. W. Frean, Commentary on the Catechism.

The Communicant is united with Christ.  
In the soul of one who worthily receives this sacrament, its effect is to bring about the union of that man with Christ, as He himself says: "He that eateth my flesh and drinketh my blood abideth in me and I in him." (John 6:56.) And since man is incorporated with Christ and united to His members through grace, it follows that through this sacrament grace is increased in those who receive it worthily.   St. Thomas Aquinas.