What does Ephesians 1:9 mean?

Part of God's wisdom (Ephesians 1:8) was to reveal or "make known" to believers "the mystery of his will." Paul uses "mystery" more in Ephesians than anywhere else in Scripture (Ephesians 3:3, 4, 6, 9; 5:32; 6:19). This concept is often misunderstood. Paul's focus is not on some mystical or self-contradictory approach, but simply on something which had been unknown in the past. In this case, a mystery of the past has now been revealed, with Gentiles coming to faith through the coming and resurrection of Christ (1 Corinthians 15:3–11).

This revealed mystery is also "according to his purpose." Paul had already referred to "purpose" in verse 5 and will mention it again in verse 11. In Ephesians 3:11 Paul talks about God's "eternal purpose," while in Ephesians 6:22 Paul speaks of his own purpose in sending Tychicus to encourage his readers. God's purpose referred to in Ephesians 3:9 was "set forth in Christ." It was not of human creation or wisdom. Only God could design and implement such an amazing, well-orchestrated plan to bring forth His plans and ultimately bring greater glory to Himself.