This was according to the eternal purpose that he accomplished in Christ Jesus our Lord,

In one death on the cross, several things were accomplished. But Paul is picking on the ultimate purpose here, meaning that the purpose of God accomplished on the cross was beyond you and me, though it includes you and me; it is beyond “the kingdom of the world” becoming “the kingdom of our Lord and of His Christ” (Revelation 11:15), but it includes that. It's like the wheel within the wheel in Ezekiel's vision.

Only God knows what Ezekiel saw. But whatever he saw, it seems that putting it into words only confused us more, but he tried his best. He was describing something we have no conception of.

He was describing one wheel inside of which were incorporated many wheels; some will see one wheel (so to speak), and that forms the basis of their systematic theology. It gave them hope and peace; another will see another wheel, and they say they cannot imagine anything making more sense because of how they have been helped.

But Paul is hinting at something here that we do not have much emphasis on in the Bible, and there is a limitation, therefore, in how much we can grasp it.

The mysterious things belong to God (Deuteronomy 29:29), and he has disclosed some of them to us. Paul calls it the incomparable riches of His grace in Christ (Ephesians 2:7), which invite endless exploration.

Paul called himself the master builder (1 Corinthians 3:10), hinting at the fact that he has been given something, a revelation, an insight that others do not have, and based on his insight, he said he has laid the foundation (1 Corinthians 3:11); therefore, we must always go back to Paul.

But in this verse, he is not giving us too much to go on. He only said God wants to disclose something to rulers and authorities in the heavenly realms.

In another place, he said he heard inexpressible things that he is not permitted to tell (2 Corinthians 12:4), so we know that some things drive him that he cannot mention.

So whatever we think was accomplished in Christ, there is much more. It seems that the resurrection was an eternal climax, and the church, the body of Christ formed from that, is the lecture for rulers and authorities in the heavenly realm.

Whatever it is, through the church, God wants to update the knowledge about Himself to those unseen entities.

The problem arises when we falsely claim to know exactly what Paul is saying. We do not, and that is okay.

But God is doing something grand with you. The recipients of this letter may think they are insignificant in the scheme of things, but they are not; they are relevant to God's eternal purpose as part of the global church. That is a powerful truth that should comfort us.

Paul is saying it is beyond some people going to heaven and others going to hell eternally, even though that is part of it.

Whatever has been disclosed is what we are expected to know.

I say all that to say thay we are supposed to be content with what we have and not pursue extra-biblical revelation in the name of a new thing, or what is called “what the Holy Spirit is saying for now,” so that we are not swept away in the error of the ungodly (Jude 1:4), so that we do not continue to be tossed to and fro by every wind of doctrine (Ephesians 4:14).

The psalmist says:

That sense of contentment in the scriptures is necessary. If all the churches in Ephesus, for example, had only this letter sent to them, they should have been content, but WE have much more; what else are we looking for? We should not make ourselves prey because of a lack of the foundation of the sufficiency of the scriptures.

If Jesus leaned on scriptures, why not we? And where we do have only a hint in the scriptures, we should leave it at that level.