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.
Source: Tractates on the Gospel of John (New Advent)