This came into effect when the times had been fulfilled, to bring together all things under Christ, the things in the heavens and the things on the earth; yes, in him! (Ephesians 1:10)
The Blessed Theodoret of Cyrus (d. ca 457AD) commenting on St. Paul’s Letter to the Ephesians, writes:
“And he made him head over all things for the Church, which is his body (Eph. 1:22-23):
Christ the Lord has the position of head, whereas those who believe in him have the position of the body. The fullness of him who fills all in all. By Church he refers to the collection of the faithful; he called it Christ’s body and the Father’s fullness: he filled it with all kinds of gifts, he dwells in it and walks about in it, according to the inspired text.” (Commentary on the Letters of St. Paul: Volume Two, pgs. 37-38)
Elsewhere, Theodoret says:
“Again he called Christ the Lord head, and the structure of the Church body, using all the terms metaphorically: just as in the case of the body the brain is the root of the nerves, and through the nerves the body has the senses, so from Christ the Lord the body of the Church receives both the founts of instruction and the basis of salvation. What sinews are in the body apostles, prophets and teachers are in the structure of the Church.” (Commentary of the Letters of St. Paul: Vol. 2, pg. 96)
Everything is to be united in Christ, who holds all things together – both things in heaven and on earth. Christ brings heaven to earth, but also earth to heaven.