55 For my flesh is meat indeed: and my blood is drink indeed.
Hilary: By the declaration of our Lord Himself, and by the teaching of our own faith, the flesh is really flesh, and the blood really blood. This then is our principle of life. While we are in the flesh, Christ dwells in us by His flesh. And we shall live by Him, according as He lives.
Augustine: Whereas men desire meat and drink to satisfy hunger and thirst, this effect is only really produced by that meat and drink, which makes the receivers of it immortal and incorruptible; i.e. the society of Saints, where is peace and unity, full and perfect. On which account our Lord has chosen for the types of His body and blood, things which become one out of many. Bread is a quantity of grains united into one mass, wine a quantity of grapes squeezed together.
56 He that eateth my flesh, and drinketh my blood, abideth in me, and I in him.
Augustine: To partake of that meat and that drink, is to dwell in Christ and Christ in you.
Chrysostom: Or, having given a promise of eternal life to those that eat Him.
Augustine: As for those, as indeed there are many, who either eat that flesh and drink that blood hypocritically, or, who having eaten, become apostates, do they dwell in Christ, and Christ in them? Nay.
He that dwells not in Christ, and in whom Christ dwells not, neither eats His flesh, nor drinks His blood: but rather eats and drinks the sacrament of it to his own damnation.
57 As the living Father hath sent me, and I live by the Father; so he that eateth me, the same also shall live by me.
Augustine: The Son does not grow better by partaking of the Father, as we do by partaking of the Son, i.e. of His one body and blood, which this eating and drinking signifies. So that His saying, I live by the Father, because He is from Him, must not be understood as detracting from His equality. Nor do the words, Even he that eateth Me, the same shall live by Me, give us the equality that He has. He does not equalize, but only mediates between God and man.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. IV (London: Rivington, 1845).