John Chrysostom

"For we, who are many, are one bread, one body." "For why speak I of communion?" saith he, "we are that self-same body." For what is the bread? The Body of Christ. And what do they become who partake of it? The Body of Christ: not many bodies, but one body. …

Homily on 1 Corinthians 24 ↗

Theophylact of Ohrid

That is why it also constitutes a unity. How then shall we not preserve love and not be in union because of this? God gives us His Body precisely in order to unite us both with Himself and with one another. Since the original nature of the flesh has been corrupted by sin and has lost life, He gave us His own flesh, sinless and life-giving, yet similar to ours, so that by partaking of it we might be mingled with it and live, as far as possible, without sin.

Commentary on 1 Corinthians ↗

Augustine of Hippo

The one who is properly said to eat the body of Christ and to drink his blood is the one who is incorporated into the unity of his body. Heretics and schismatics can receive the sacrament but to no avail—in fact, to their harm—since the result is to increase their pain rather than to curtail the length of their punishment.

City of God 21.25 ↗

Ambrose of Milan

Wherefore every soul which receives that bread which comes down from heaven is the house of bread, that is, the Bread of Christ, being nourished and supported and having its heart strengthened by that heavenly bread which dwells within it. Hence Paul also says, For we being many are one bread. Every faithful soul is Bethlehem, as Jerusalem also is said to be, which has the peace and tranquillity of that Jerusalem which is above, in heaven. That is the true Bread which, when broken into pieces, fed all men.

Letter 63 ↗