18 In a word, He now explains how that which He speaks of comes to pass, and what it is to eat His body and to drink His blood. “He that eats my flesh, and drinks my blood, dwells in me, and I in him.” This it is, therefore, for a man to eat that meat and to drink that drink, to dwell in Christ, and to have Christ dwelling in him. Consequently, he that dwells not in Christ, and in whom Christ dwells not, doubtless neither eats His flesh [spiritually] nor drinks His blood [although he may press the sacrament of the body and blood of Christ carnally and visibly with his teeth], but rather does he eat and drink the sacrament of so great a thing to his own judgment, because he, being unclean, has presumed to come to the sacraments of Christ, which no man takes worthily except he that is pure: of such it is said, “Blessed are the pure in heart, for they shall see God.”
19. “As the living Father has sent me,” says He, “and I live by the Father; so he that eats me, even he shall live by me.” He says not: As I eat the Father, and live by the Father; so he that eats me, the same shall live by me. For the Son, who was begotten equal, does not become better by participation of the Father; just as we are made better by participation of the Son, through the unity of His body and blood, which thing that eating and drinking signifies. We live then by Him, by eating Him; that is, by receiving Himself as the eternal life, which we did not have from ourselves. Himself, however, lives by the Father, being sent by Him, because “He emptied Himself, being made obedient even unto the death of the cross.” For if we take this declaration, “I live by the Father,” according to that which He says in another place, “The Father is greater than I;” just as we, too, live by Him who is greater than we; this results from His being sent. The sending is in fact the emptying of Himself, and His taking upon Him the form of a servant: and this is rightly understood, while also the Son's equality of nature with the Father is preserved. For the Father is greater than the Son as man, but He has the Son as God equal—while the same is both God and man, Son of God and Son of man, one Christ Jesus. To this effect, if these words are rightly understood, He spoke thus: “As the living Father has sent me, and I live by the Father; so he that eats me, even he shall live by me:” just as if He were to say, My emptying of myself (in that He sent me) effected that I should live by the Father; that is, should refer my life to Him as the greater; but that any should live by me is effected by that participation in which he eats me. Therefore, I being humbled, do live by the Father, man being raised up, lives by me. But if it was said, “I live by the Father,” so as to mean, that He is of the Father, not the Father of Him, it was said without detriment to His equality. And yet further, by saying, “And he that eats me, even he shall live by me,” He did not signify that His own equality was the same as our equality, but He thereby showed the grace of the Mediator.
20. “This is the bread that comes down from heaven;” that by eating it we may live, since we cannot have eternal life from ourselves. “Not,” says He, “as your fathers did eat manna, and are dead: he that eats this bread shall live forever.” That those fathers are dead, He would have to be understood as meaning, that they do not live forever. For even they who eat Christ shall certainly die temporally; but they live forever, because Christ is eternal life.
Source: Tractates on the Gospel of John (New Advent)