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.

I think it will work best if we put verses 9 and 10 together. God, in revealing his message of grace and salvation, is making known to us the mystery of His will. A mystery, the way Paul uses the term, is something that God has kept unrevealed until a particular time in history. When that time arrives, He makes known that mystery to His prophets and apostles, who then incorporated the truth in the Scriptures.

Paul writes in Colossians 1:25 “I became a minister according to the stewardship from God that was given to me for you, to make the word of God fully known, the mystery hidden for ages and generations but now revealed to his saints.”
The God of the universe has a plan which He is gradually putting into play. And He, the God of the universe, has chosen to reveal that mystery to human beings. That is an amazing thing to contemplate.

Paul goes on to tell us in Ephesians 1:9 , that He is making this mystery known according to His purpose. So even the revealing is according to a purpose. And remember, God doesn’t change his plans. If He does something according to His purpose, it is a purpose He has always had in mind for the zillions of years of eternity past.

This purpose is set forth in Christ or displayed or revealed in Christ. Further, it is a plan for the fullness of time. When everything surges toward the conclusion, what is that plan, Paul? That purpose is to unite everything in heaven and on earth in Christ. Everything will be brought together in Christ. Remember that the word “Christ” means Messiah and that is none other than Jesus. And John told us that Jesus is the Word, the logos of God.
Logos means more than “word.” Logos, the way it was used in the Greek culture at Jesus time, meant the wisdom, logic, thought, and, yes, word, behind all of creation. John told us that this logos became a man and lived here on earth among us.

So in the end, everything will be brought together in Christ.
This is the mystery that had been hidden for thousands of years, but which God revealed to His apostles 2000 years ago.

Exported from Logos Bible Software, 10:47 AM September 22, 2020.