The Real Presence of Jesus Christ in the Sacrament of the Eucharist.
The Lord Jesus, on the night before he suffered on the cross, shared one last meal with his disciples. During this meal our Savior instituted the sacrament of his Body and Blood. He did this in order to perpetuate the sacrifice of the Cross throughout the ages and to entrust to the Church his Spouse a memorial of his death and resurrection. As the Gospel of Matthew tells us:
While they were eating, Jesus took bread, said the blessing, broke it, and giving it to his disciples said, "Take and eat; this is my body." Then he took a cup, gave thanks, and gave it to them, saying, "Drink from it, all of you, for this is my blood of the covenant, which will be shed on behalf of many for the forgiveness of sins." (Mt 26:26-28; cf. Mk 14:22-24, Lk 22:17-20, 1 Cor 11:23-25)
Recalling these words of Jesus, the Catholic Church professes that, in the celebration of the Eucharist, bread and wine become the Body and Blood of Jesus Christ through the power of the Holy Spirit and the instrumentality of the priest. Jesus said: "I am the living bread that came down from heaven; whoever eats this bread will live forever; and the bread that I will give is my flesh for the life of the world. . . . For my flesh is true food, and my blood is true drink" (Jn 6:51-55). The whole Christ is truly present, body, blood, soul, and divinity, under the appearances of bread and wine—the glorified Christ who rose from the dead after dying for our sins. This is what the Church means when she speaks of the "Real Presence" of Christ in the Eucharist. This presence of Christ in the Eucharist is called "real" not to exclude other types of his presence as if they could not be understood as real (cf. Catechism, no. 1374). The risen Christ is present to his Church in many ways, but most especially through the sacrament of his Body and Blood.
What does it mean that Jesus Christ is present in the Eucharist under the appearances of bread and wine? How does this happen? The presence of the risen Christ in the Eucharist is an inexhaustible mystery that the Church can never fully explain in words. We must remember that the triune God is the creator of all that exists and has the power to do more than we can possibly imagine. As St. Ambrose said: "If the word of the Lord Jesus is so powerful as to bring into existence things which were not, then a fortiorithose things which already exist can be changed into something else" ( De Sacramentis, IV, 5-16). God created the world in order to share his life with persons who are not God. This great plan of salvation reveals a wisdom that surpasses our understanding. But we are not left in ignorance: for out of his love for us, God reveals his truth to us in ways that we can understand through the gift of faith and the grace of the Holy Spirit dwelling in us. We are thus enabled to understand at least in some measure what would otherwise remain unknown to us, though we can never completely comprehend the mystery of God.
~from the United States Conference of Catholic Bishops
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