Ephesians 3:3-6

Paul reemphasized the mystery in the relationship of Gentile Christians to Jews in Ephesians 3:6

• Coheirs. Neither Jews nor Gentiles possessed privilege over the other. All were sinners and all were saved the same way and receive the same inheritance. This was a mystery!
• Members of the same body. God formed them into one body, a new body like no one had ever seen. This was a mystery! 
• Partners in the promise in Christ Jesus through the gospel. Both Jews and Gentiles shared in the gift of the Holy Spirit as equals. This had amazed Peter when Cornelius and other Gentiles received the Holy Spirit the same way the apostles had (Acts 10:47-48). This was a mystery!

Ephesians 3:7-9

Here are some of the unsearchable, incalculable riches (Ephesians 3:8) of the gospel that are still unfathomable (a mystery).

Ephesians 3:10

The mouthpiece of the gospel is the church. When we demonstrate our oneness in Christ and share the gospel with others, the mystery is being proclaimed to heavenly authorities (i.e. both holy and demonic angels).

• Fallen Angels/Demons: Oddly enough, fallen angels glorify God through their failures! God receives glory every time their plans fail. This reveals the greatness of God and the futility of their evil intentions to destroy His church.
• Holy Angels: 1 Peter 1:12 states, “These things have now been announced to you through those who preached the gospel to you by the Holy Spirit sent from heaven—angels long to catch a glimpse of these things.”

John MacArthur describes this amazing truth about holy angels: “The angels can see the power of God in creation, the wrath of God at Sinai, and the love of God at Calvary. But above all they see His manifold [multi-colored, multi-faceted] wisdom that is made known through the church. They see Him taking Jew and Gentile, slave and free, male and female—who together murdered the Messiah and were worthy only of hell—and making them, by that very cross of murder, one spiritual Body in Jesus Christ. They see Him breaking down every barrier, every wall that divides and making all believers one in an indivisible, intimate, and eternal union with the Father, the Son, the Holy Spirit, and every other believer from every other age and circumstance. "There is joy in the presence of the angels of God over one sinner who repents," Jesus said (Luke 15:10). Every sinner who repents and turns to Christ adds another spiritual stone to God's temple, another member to His Body, and becomes another forgiven and cleansed sinner who is made eternally one with every other forgiven and cleansed sinner. The holy angels not only are interested in the salvation of men (1 Pet. 1:12) but constantly watch the face of God in heaven to see His reaction to the treatment of His saved earthly children (Matt. 18:10, 14), standing ready to carry out any mission in their behalf."

-- [MacArthur, John (1985). MacArthur New Testament Commentary - Ephesians (p. 85). WORDsearch. Retrieved from https://app.wordsearchbible.com.]

Ephesians 3:11-13