After the priest performs the consecration, the bread and wine retain the same appearance but their substance has changed. The believer who feeds on the "bread of life" is deeply transformed by God.
The Eucharist is Jesus Christ himself: His Body, Blood, Soul, and Divinity. Therefore, the whole Christ is present in the consecrated bread and wine.
The Eucharist is Jesus Christ Himself. At the heart of the celebration of the Mass, the priest invokes the Holy Spirit and repeats the words of Jesus at the Last Supper: "This is my body which is given up for you," and "this is my blood which will be shed for you and for many." At that moment, the bread is mysteriously changed into the body of Christ and the wine into His blood, though the shape, color, smell, and taste of the bread and wine remain unchanged. In this manner, the whole substance of the bread is converted into the substance of Christ's body, and the whole substance of the wine becomes the substance of his blood.
Put another way, while the substance (inner reality) is changed, the appearance of the hosts (an accidental quality) remains those of bread and wine, and does not become those of the human body. That is why the presence of God remains veiled . Quoting St. Thomas Aquinas, the Catechism of the Catholic Church states, "we do not learn through the senses, but by faith alone." This presence, as unobtrusive as it is powerful, has inspired many works of art. It has amazed and filled millions of people over the last two millennia, starting from Jesus' Last Supper with his disciples before his death and resurrection. The American Protestant pastor Scott Hahn, who later converted to Catholicism, was stunned to discover that presence. He tells the story in these words: "I saw the priest raise that white host, and I felt a prayer rise up in my heart as a whisper: ‘My Lord and my God, it's really you!’”
Jesus Christ, living and glorious, becomes present in the world today in many ways: in his Word, in his Church's prayer, "where two or three are gathered in my name" (Mt. 18:20), in the poor, the sick, the imprisoned, in the sacraments, and in the person of the priest who acts in his person. But at the highest point, substantially, Jesus Christ is present under the species of consecrated wheat bread and grape wine.
Christ's presence is real, though not apparent. Jesus Himself said so, and the Catholic Church has continued to reaffirm it, always aware of the importance of this controversial truth of faith.
That God should be in a piece of bread and that men should eat him has always been an object of scandal. Ever since Jesus first proclaimed this teaching, many disciples left him. And throughout the history of Christianity, the real presence of Christ in the Eucharist has been questioned, creating many heresies and divisions. But Jesus and the Catholic Church have firmly upheld it, against all odds, as a central element of faith. In Chapter 6 of St. John's Gospel, Jesus reaffirms, "My flesh is real food and my blood is drink indeed." In the 16th century, the Council of Trent affirmed that Christ is present in the Eucharist "truly, really, and substantially," rejecting the idea that this sacrament is simply a symbol or sign of a body that is absent.
In the splendid hymns of poets and composers, the body of Christ present on the altar in the monstrance and tabernacle is adored by millions of people as "true" ("verum corpus") to distinguish it from a body that is merely "apparent" or symbolic, and also to distinguish it from the "mystical" body that is the Church.
By virtue of His real presence, which is unique and mysterious under the appearance of consecrated bread and wine, Jesus eminently fulfills His promise to be with us always until the end of the world. The Gospel of St. Matthew concludes with that promise.
In the consecrated bread, Christ gives men food that transforms their existence and anticipates their life in and with God.
Just as the grain of wheat is placed in the ground and dies, bringing forth a new stalk with which bread will be made, so Jesus gives Himself totally so that each person may have access to new, eternal life. He did so in Jerusalem around the year 30 AD and renews it in each consecration of the bread. This is a total gift of love, a sacrifice to inaugurate the passage from death to life.
Thus, says Pope Benedict XVI, the Eucharist appears as "the great and permanent encounter of God with man, in which the Lord gives Himself as 'flesh' so that in Him and walking in His way, we may become 'spirit.' Just as Christ was transformed by the cross so as to enter into a new form of humanity and corporeity — a form fully associated with the nature of God — so this food must open our existence. It is a passage through the cross and an anticipation of the new life of living in and with God."
Lifted up and saved from their daily falls and united in communion with God and among themselves, those who feed on the unique "bread of life" are eternally the very Body of Christ. Thus, the real presence of Christ in the Eucharist anticipates the final divine presence, in which they will rejoice after physical death when they pass on to the Father.