John Chrysostom

"Which is His body." In order then that when you hear of the Head you may not conceive the notion of supremacy only, but also of consolidation, and that you may behold Him not as supreme Ruler only, but as Head of a body. "The fulness of Him that filleth all in all" he says. As though this were not sufficient to show the close connection and relationship, what does he add? "The fullness of Christ is the Church." And rightly, for the complement of the head is the body, and the complement of the body is the head. …

Homily on Ephesians 3 ↗

Theophylact of Ohrid

Lest you, having heard this, should think that by the name "head" some rule and authority is meant (for the word "head" has this meaning as well), he says that Christ is the head of the Church as a body, and is likewise akin to it and closely united with it. And the Church is His fullness. …

Commentary on Ephesians ↗

Theodoret of Cyrus

By the church he means the whole community of the faithful. This he calls the body of Christ and the fullness of the Father. This body he has filled with all gifts. He "lives in it and goes about in it," as the voice of prophecy says. But this will be more strictly so in the future life.… In the present life God is in all, since his nature is uncircumscribed; but he is not "all in all," since some are impious and some lawless. Yet he lives in those who fear him and who put hope in his mercy. …

Interpretation of the Epistle to the Ephesians 1.23 ↗

Thomas Aquinas

He speaks of the relation of the Church to Christ at which is his body, inasmuch as she is subject to him, receives his influence, and shares the same nature with Christ. "Just as the body is one and has many members, and all the members of the body, though many, are one body, so it is with Christ. For in one Spirit were we all baptized into one body" (1 Cor. 12:12-13). He explains which is his body by adding the fulness of him. …

Commentary on Ephesians ↗