The liturgical life of the Church revolves around the sacraments, with the Eucharist at the center (National Directory for Catechesis, #35). At Mass, we are fed by the Word and nourished by the Body and Blood of Christ. We believe that the Risen Jesus is truly and substantially present in the Eucharist. The Eucharist is not a sign or symbol of Jesus; rather we receive Jesus himself in and through the Eucharistic species. The priest, through the power of his ordination and the action of the Holy Spirit, transforms the bread and wine into the Body and Blood of Jesus. This is call transubstantiation.
By the consecration the transubstantiation of the bread and wine into the Body and Blood of Christ is brought about. Under the consecrated species of bread and wine Christ himself, living and glorious, is present in a true, real, and substantial manner: his Body and his Blood, with his soul and his divinity. (CCC 1413)
In the gospels we read that the Eucharist was instituted at the Last Supper. This is the fulfillment of the covenants in the Hebrew Scriptures. In the Last Supper narratives, Jesus took, broke and gave bread and wine to his disciples. In the blessing of the cup of wine, Jesus calls it “the blood of the covenant” (Matthew and Mark) and the “new covenant in my blood” (Luke).
This reminds us of the blood ritual with which the covenant was ratified at Sinai (Ex 24) -- the sprinkled the blood of sacrificed animals united God and Israel in one relationship, so now the shed blood of Jesus on the cross is the bond of union between new covenant partners -- God the Father, Jesus and the Christian Church. Through Jesus’ sacrifice, all the baptized are in relationship with God.
The Catechism teaches that all Catholics who have received their First Holy Communion are welcome to receive Eucharist at Mass unless sin a state of mortal sin.
Receiving the Eucharist changes us. It signifies and effects the unity of the community and serves to strengthen the Body of Christ.