From The Gospel of John, by Francis Martin and William Wright, commenting on John 6:51-58
Jesus’ words about his flesh and blood have a strong realism. The verb used for “eat” in 6:54–58 is different from the verb used in the preceding conversation and is very graphic. In other Greek literature, it designates how animals eat.
While obedient listening and faith are means of ingesting God’s Word and wisdom, the change to a more concrete verb for eating accents the fact that Christ’s offer of his body and blood entails something even more radical: consuming his flesh and blood in the Eucharist. All the material food and drink in the world, including the manna and the multiplied loaves, are gifs from God to sustain mortal life. They are also imperfect foreshadowings of the true food and true drink by which God gives us eternal life.
Jesus explains the Eucharist as the food of eternal life by linking it to participation in the divine communion: Whoever eats my flesh and drinks my blood remains in me and I in him. In John, the verb “remain” (menō) designates the mutual indwelling of Father and Son, the eternal relationship between them in which Jesus invites his disciples to share (see 1:39; 14:10; 15:4–10). By our consuming Christ’s body and blood in the Eucharist, he dwells within us, and we in turn share in his divine life. The Eucharist is truly “holy communion” (Catechism 1391). As St. Paul writes to the Corinthians, “The cup of blessing that we bless, is it not a participation [Greek koinōnia, literally, “fellowship, communion”] in the blood of Christ? The bread that we break, is it not a participation in the body of Christ?” (1 Cor 10:16).
Jesus continues, Just as the living Father sent me and I have life because of the Father, so also the one who feeds on me will have life because of me. In 5:26, Jesus spoke of his own possession of the divine life, which is the Father’s eternal gift to the Son. As the bread of life, Jesus came down from heaven to give “life to the world” (6:33). Jesus’ divine life is given to those who receive Jesus in faith as God’s wisdom and, even more profoundly, consume his Eucharistic body and blood.