Christ invites us to become His friends by eating with Him. When did Christ institute this special meal? The Gospel teaches us that before His passion, Christ had His last supper with His apostles. It was then that Christ took bread, gave thanks, and broke it, saying, "This is my Body..." and he also took the cup, saying, "This is my Blood..." So the Catholic Church teaches that Christ intended this, namely to make Himself food and drink for those who believe in Him. According to Jewish thought, the body is the person and the blood is the source of life that feeds the person. So when Jesus said "This is my body....This is my blood...", he meant, "This is myself". Christ gives Himself completely to us. The bread and wine are transformed into His own Body and Blood by the power of the Holy Spirit through the words of the Word of God spoken by the priest. This change is known as "transubstantiation", which means: the substance of the bread and wine are transformed into the Body and Blood of Christ (cf. CCC 1376), although the appearance remains that of bread and wine.
Christ himself said, "I am the living bread that came down from heaven. If anyone eats of this bread, he will live forever; and the bread that I give is My flesh, which I will give for the life of the world... Truly I tell you, unless you eat the flesh of the Son of Man and drink His blood, you have no life in you. Whoever eats My flesh and drinks My blood has eternal life, and I will raise him on the last day. For My flesh is true food, and My blood is truly drunk. He who eats my flesh and drinks my blood abides in me, and I in him. Just as the living Father sent me and I live by the Father, so he who eats me will live by me. This is the bread that came down from heaven, not as your fathers ate and died. Whoever eats this bread will live forever." (Jn 6:51-58) Because in this meal, the meal is Christ, the Bread of life who came down from heaven, this meal is heavenly. Also, since what we eat in this meal is Christ, "the Holy One of God" (Mk 1:24), it is also called a holy meal. Moreover, it is called a holy meal because we who receive it must be at peace with God, that is, not in a state of mortal sin (cf. CCC 1385), and because through the Eucharistic holy meal the Lord Jesus in His way sanctifies us who receive Him.
Perhaps we have heard how people question the Eucharist and find it strange that Jesus commands us to eat His Body and drink His Blood. If many people don't understand this now, it's not surprising, because, from the very beginning when Jesus gave this teaching, many people said, "This is a hard saying; who can hear it?" (Jn 6:60) So many of them withdrew and no longer followed Jesus (cf. Jn 6:67). Yet Jesus did not change His teaching. He did not say, "Wait a minute, I don't mean this is My Body, but this symbolizes My Body..." Instead, He asked His apostles, "Don't you want to go too?" Peter answered Him: "Lord, to whom shall we go? Your word is the word of eternal life; and we have believed and know that you are the Holy One of God." (Jn 6:61-69)
Here the Apostle Peter does not say that he understands how bread and wine can turn into the Body and Blood of Christ. Rather he simply accepted the authority of Jesus' words and believed in the power of Jesus to do so, for the words of Christ are the words of eternal life. The Apostle Paul also believed that in the Eucharist, Christ is truly present, so he said, "Whoever unworthily eats the bread or drinks the cup of the Lord sins against the body and blood of the Lord....whoever eats and drinks without recognizing the body of the Lord brings judgment on himself" (1Cor 11:27,29). So, we Catholics, like the Apostles Peter and Paul, accept what Jesus said as truth because His words are words of eternal life. Although we cannot understand how the bread and wine could be transformed into the Body and Blood of the Lord Jesus, we accept by faith that Christ, by the power of His Holy Spirit, made this transformation, to give Himself spiritual food for us.
St. John Paul II said, "The Eucharist is the true banquet, in which Christ offers Himself as a meal that strengthens us." ((Pope John Paul II, Ecclesia de Euscharistia, 16)) Thus, the first churches also called the Eucharist the Lord's Supper (1Cor 11:20). Christ Himself declared that His Body is true food and His Blood is truly drinking (cf. Jn 6:55) because He wanted us to relate the Eucharist to our daily food and drink. Just as food and drink can strengthen our bodies and become one with our bodies; so too the Eucharist can strengthen us, so that we can become like Him whom we receive.
By receiving Christ, we also receive His divine life. Christ said, "Abide in me and I in you. Just as the branch cannot bear fruit of itself unless it abides in the vine, neither can you unless you abide in me. I am the vine and you are the branches. He who abides in me and I in him bears much fruit, for apart from me you can do nothing." (Jn 15:4-5). The Eucharist is a way for us to abide in Him, receiving divine life so that we can grow and bear fruit. For Jesus said, "Whoever eats my flesh and drinks my blood abides in me and I in him." For just as Christ lives by the Father, so we who eat He will also live by Christ (cf. John 6:57).