I am the living bread that came down from heaven; whoever eats this bread will live forever. Whoever eats my flesh and drinks my blood has eternal life and remains in me and I in him. (John 6:51, 54, 56)
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 (Exodus 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 Eucharist are welcome to receive Eucharist at Mass unless in a state of mortal sin.