The sacrament of Holy Communion nourishes the spiritual life of the believer through the Real Presence of Jesus in the Eucharist.
Jesus said, “I am the bread of life. Your fathers ate the manna in the wilderness, and they died. This is the bread which comes down from heaven, that a man may eat of it and not die. I am the living bread which came down from heaven; if any one eats of this bread, he will live for ever; and the bread which I shall give for the life of the world is my flesh.” The Jews then disputed among themselves, saying, “How can this man give us his flesh to eat?” So Jesus said to them, “Truly, truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you; he who eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day. For my flesh is food indeed, and my blood is drink indeed. He who eats my flesh and drinks my blood abides in me, and I in him. As the living Father sent me, and I live because of the Father, so he who eats me will live because of me. This is the bread which came down from heaven, not such as the fathers ate and died; he who eats this bread will live for ever.” 1
The Blessed Sacrament
The Lord instituted this sacrament during the Passover meal before His crucifixion.
Now as they were eating, Jesus took bread, and blessed, and broke it, and gave it to the disciples and said, “Take, eat; this is my body.” And he took a cup, and when he had given thanks he gave it to them, saying, “Drink of it, all of you; for this is my blood of the covenant, which is poured out for many for the forgiveness of sins. I tell you I shall not drink again of this fruit of the vine until that day when I drink it new with you in my Father’s kingdom.” 3
The Eucharist is a re-presentation of the once-and-for all, perfect, and complete sacrificial atonement of Christ, which satisfied the penalty of sins of the world. This mystery is transcendent of time and space and is not confounded by His high priestly presence in heaven. The first Eucharistic meal looked forward in time to the crucifixion, and subsequent Eucharistic meals look backward in time.4
One must be in a state of grace (free of mortal sin) in order to receive Holy Communion. And since it is not only a communion with Christ, but also communion with the Church, one must also be Catholic to receive Holy Communion.