Read Ephesians 3:1-21
Paul talks about a "mystery" in Ephesians three which was revealed to him and the other apostles and prophets of that time, that was hidden from the "sons of men" (children of Israel) that went before them. The mystery of course was that the Gentiles (all those who weren't Jewish) would now, because of Christ, be partakers in the same blessings God had promised the Jews (Ephesians 3:6-7). In Ephesians chapter two, Paul goes into great detail on what that unity for humanity now meant.
Paul believed (and rightly so) that he was called specifically to preach this message of unity to all Gentiles (Ephesians 3:8). This was a unique ministry for him in particular, because he was formerly a very religious, dogmatic Jew. Clearly, his experience of meeting Jesus on the road to Damascus changed his life forever. God had taken the most unlikliest of men to preach a new message of redepemption that was now inclusive. No one was left out of God's plan for humanity. All were now given the same opportunities, the same blessings, the same promises as the Jews. This was the mystery of Christ that was revealed to Paul and the other apostles.
Paul also believed that the mystery of Christ was something that, when accepted, created a unique fellowship or community of believers, who then had a specific responsibility to make the mystery known. Some have implied that Ephesians 3:10 means the church has authority over "rulers and authorities" in heavenly places. But a careful examination of the text reveals something far different. And we also discover once again, how the NIV has distorted scripture. This is what Ephesians 3:10 says in the KJV:
To the intent that now unto the principalities and powers in heavenly places might be known by the church the manifold wisdom of God,
This is how the NIV translates Ephesians 3:10:
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.
You can see how the NIV has added words to the text and twisted them to mean something that seems to imply that the church is making the wisdom of God known to "spiritual" beings in the heavenly places. This is just not so and it is inconsistent with the surrounding text.
The word for "principalities" that Paul used is archē which means "a beginning" and the word used for "powers" is exousia and means "authority". In other words, what Paul is saying here is that the manifold wisdom of God that begins and has authority in the heavenly places is now made known to the church. For anyone to translate this as to mean that now the church was teaching heavenly beings or had power over them, would be inconsistent with what Paul has written so far in this letter. For he is trying to stress to the Gentiles the privileges they (all believers) now have in Christ. Imagine! Because of Jesus, we have access not only to God, but He reveals His wisdom and shares it with us. How? Through His Holy Spirit! 1 Corinthians 2:7-16 further illustrates this point. The mystery of God is that we, through His Holy Spirit, now have the mind of Christ, and with that comes knowledge and wisdom to discern Scripture and have deep truths revealed to us, which those without the Holy Spirit are not privy to.
And so Paul ends Ephesians three with a heartfelt prayer for the Ephesians that we can all take to heart - that God would strengthen them, through His Holy Spirit. That Christ would dwell in their hearts through faith and that they would be rooted and grounded in love, able to comprehend the extent of God's love for them and to know the love of Christ, so that they may be filled with all the fullness of God (Ephesians 3:14-19). And through the mystery of His Holy Spirit we are all united with the same purpose - to grow in faith and in service to Jesus Christ our Lord.