Ephesians 1:10 is the verse where the apostle Paul tells us what everything is all about. And everything means everything:
“…making known to us the mystery of his will, according to his purpose, which he set forth in Christ as a plan for the fullness of time, to unite all things in him, things in heaven and things on earth.” (Ephesian 1:9-10, ESV)
This is what Paul saw with his heart. This is what encouraged him in all of his suffering. It’s the vision he labored and prayed to impart to others.
God has a plan to unite all things in Christ.
The more clearly we see this, the better off we will be.
So what does it mean to “unite all things in Christ”?
Attempts to translate this into English have yielded some beautiful language:
God is going to “gather together in one all things in Christ.” (NKJV)
It is His plan “that all human history shall be consummated in Christ.” (PHILLIPS)
In other words, God’s purpose is “the summing up of all things in Christ” (NASB)
“Summing up” looks like a good term to describe what Paul is talking about. According to Stephen E. Fowl’s commentary on Ephesians, the word Paul used normally applied to a public speaker bringing his speech to a close:
The verb used here (ἀνακεφαλαιώσασθαι) is often used to indicate the drawing together of discrete points into a summative argument.
Fowl then quickly surveyed some of church fathers’ interpretations of Ephesians 1:10. Such interpretations include all things being restored and renewed and made subject to the lordship of Christ.
He then added this:
What all of these early interpreters seem to be doing is explicating a term that normally applies to rhetoric—summing up points of an argument—in the light of Paul’s use of that term to describe Christ’s role in all of history.
God certainly is going to renew all things in Christ and bring all things under Christ’s authority. But we learn that from other scriptures, not necessarily from this big word in Ephesians 1:10.
I think Paul did indeed have renewal and obedience to Christ in mind when he said that God had a plan to sum up all things in Him. But the word he used was a rhetorical term, and I find that helpful in understanding Ephesians 1:10.
Ever since God said “Let there be light,” he has been speaking. He has been speaking in many different ways, but he has always been saying the same thing. Nevertheless, he has not always been understood, and he has never been fully understood, except by the very one he was talking about.
But God is going to bring it all together. He is going to speak so clearly in Christ that what he says will shine all the way back to creation and illuminate all of history.
When Jesus returns, it will be God’s way of saying, “In closing…” Then everyone will see that God has never changed the subject, although he changed his manner of speaking.
One more thing:
If Paul is indeed using a metaphor from the world of rhetoric, he is mixing it. In Ephesians 1:9 he said that in Christ, God had “a plan for the fullness of time.”
The word plan in the ESV translates a Greek word (οἰκονομίαν) that means administration, management, oversight, or stewardship. So history is not only God giving a speech, it’s God governing and ordering things to a certain end.
 Fowl, Stephen E., Ephesians: A Commentary (2012, Westminster John Knox Press)