Ephesians 3:9-10

and to make plain to everyone the administration of this mystery, which for ages past was kept hidden in God, who created all things.

(v9, NIV)

Not only is Paul preaching to the gospel, but he has another task. The plan is to make everyone aware of the administration of the mystery. The mystery is described in verse 6. The mystery is that the Gentiles are now heirs along with the Jews. Paul is letting people know how the mystery is being given out to people. People are being made aware by Paul's preaching, but Paul is also explaining how it is happening. It was hidden from them beforehand. God hid it from them. However, now it is being made known to them through the Holy Spirit as explained in verse 5.

His intent was that now, through the church, the manifold wisdom of God should be made known to the rulers and authorities in the heavenly realms,

(v10, NIV)

Here we find God's reasoning for doing this. It's for his wisdom to be made known. But who are the rulers and authorities in the heavenly realms? I'm not sure exactly. However, since they are not in the earthly realms, they are not us actually acting it out. Therefore, they are mere spectators or people who are seeing what we are doing. So, to those who are observers and see what is happening on the earth, the wisdom of God will be made known to them through his working in us.