“making known to us the mystery of his will, according to his good pleasure that he purposed in him, for the administration of the fullness of times, to bring together all things in Christ, the things in the heavens and the things on the earth, in him” LEB
“He made known to us the mystery of His will, according to His kind intention which He purposed in Him with a view to an administration suitable to the fullness of the times, that is, the summing up of all things in Christ, things in the heavens and things on the earth. In Him” NAS
“God’s secret plan has now been revealed to us; it is a plan centered on Christ, designed long ago according to his good pleasure. And this is his plan: At the right time he will bring everything together under the authority of Christ — everything in heaven and on earth.” NLT
“The mystery of His will,” which is the “mystery of Christ” (Col 2:1,2; Col 4:3) and the salvation of the world through Him, extends to or includes or has in view the “bringing together of all things in Christ, the things in the heavens and the things on the earth, in him.”
The course of this world is moving along according to a Divine and orderly plan, and the culmination of that plan, when all has been accomplished, (“the fullness of times”), will be when all things are united in Christ. “All things” consists of everything in the heavens and on earth.
There is some debate whether “things” literally refers to things, or just another way of referring to persons. Based on what Paul says in Romans 8:18-23, I think it clearly refers to both. Those who are “in Christ” on earth now and those who are in Heaven now, are in view, as well as the holy angels. It also refers to the universe. When sin entered, first by the rebellion of angels (who became Satan and demons), then by the disobedience of man, things changed.
At that point this was no longer a pure universe. Even now Satan and his evil host of fallen angels, prowl the heavens and earth. Paul even talks about that later in his letter:
“Put on the full armor of God, so that you will be able to stand firm against the schemes of the devil. For our struggle is not against flesh and blood, but against the rulers, against the powers, against the world forces of this darkness, against the spiritual forces of wickedness in the heavenly places.” LEB (Eph 6:11,12)
We have no way of knowing the boundaries of these evil spirits, but the point is, the universe has been contaminated by their presence and by their evil works. Furthermore, it was Satan who tempted Eve, which resulted in her fall, along with Adam. And of course, their sin was passed on to the rest of us.
One day when God’s plan for mankind and for this universe has been completed, He will create a new heaven and a new earth (Rev 21:1). Those without Christ, who died in their sins, will be confined to an eternal prison, where they will be completely sealed off from the Kingdom of God where “righteousness dwells” (2 Pet 3:13). We’ll have a new universe (2 Pet 3:10) that will no longer be infected with sin and evil, but will be completely contained.
Once Christ has shut away sin and its influence, once He has destroyed His enemies, once He “has abolished all rule and authority and power” (1 Cor 15:24), once He has destroyed the last enemy, which is death (1 Cor 15:26), then He will set up His eternal Kingdom in the new heaven and earth, where everyone “in Christ” will dwell in His presence forever and ever. All of creation is subject to Christ, and being held together by Him (Col 1:16,17).
Paul discusses the “all things” of our text verse in more detail in Romans 8:18-23, and I’ll leave you with that:
“18 For I consider that the sufferings of the present time are not worthy [to be compared] with the glory that is about to be revealed to us. 19 For the eagerly expecting creation awaits eagerly the revelation of the sons of God. 20 For the creation has been subjected to futility, not willingly, but because of the one who subjected [it], in hope 21 that the creation itself also will be set free from its servility to decay, into the glorious freedom of the children of God. 22 For we know that the whole creation groans together and suffers agony together until now. 23 Not only [this], but we ourselves also, having the first fruits of the Spirit, even we ourselves groan within ourselves [while we] await eagerly [our] adoption, the redemption of our body.” LEB