What after all IS the mystery of Christ? The incarnation? His atoning death on the cross? Justification? His resurrection? All of this? What insight are we about to gain?
In verse 6 Paul writes (drumroll): that Gentiles are now fellow "heirs" and "partakers" of God's promise in Christ through the gospel.
... (crickets: chirp chirp)
Feeling let down? Probably because Gentiles take their inclusion for granted. I'm sorry, but this is actually pretty consistent. Compare the messages of Colossians and Ephesians. The message isn't strictly ordered the exact same way, but it's very close and the same elements are all there. First, (after greetings and commendations) there is a statement emphasizing the former rift or distance of his audience. Next, Christ is praised as the one who makes unity of the diverse elements and/or people of God's world, destroying obstacles to our reconciliation. Then, this is described as the essential working of the message that Paul is preaching. In each book, there is a dual-emphasis of God uniting things and people "in" Christ, and also Christ dwelling "in" his audience, which is a kind of mutual in-dwelling that theologians usually talk about with reference to the Trinity using the Greek word "perichoresis" (and you wouldn't believe what auto-spell just tried to do with that).
This is potent stuff that is de-fanged in some ways by at least 2 things (besides the Gentile assumptions already described):
1. The sense of shame that many people have about their sins and deficiencies continuing to separate them from God
2. The opposite idea that "believing" certain propositions about Jesus are the beginning and end of relationship with God.
Paul seems to want people to move toward a place of maturity and unity with God and others that leaves both of these limiting senses far behind.