“At the heart of the Eucharistic celebration are the bread and wine that, by the words of Christ and the invocation of the Holy Spirit, become Christ's Body and Blood. Faithful to the Lord's command the Church continues to do, in his memory and until his glorious return, what he did on the eve of his Passion: "He took bread... ." "He took the cup filled with wine. . . " The signs of bread and wine become, in a way surpassing understanding, the Body and Blood of Christ.”
(Catechism of the Catholic Church, 1333)
Source and Summit
The Eucharist is the “source and summit of the Christian life,” and the third sacrament of initiation. "It is the culmination both of God's action sanctifying the world in Christ and of the worship men offer to Christ and through him to the Father in the Holy Spirit." (Catechism of the Catholic Church, 1324,1325);
The Eucharist: Faith Facts
Before one receives Holy Communion, it is appropriate to bow before the Sacred Host or Chalice, because the bread and wine have truly become the Body and Blood of Jesus Christ. After the priest or minister says, “The Body of Christ” or “The Blood of Christ,” we respond, “Amen,” and receive the Body of Christ on the tongue or in the hand, consuming it immediately. The Blood of Christ we drink, holding the chalice reverently and returning it to the priest or minister.