Ephesians 3 - What is this mystery which in other ages was not made known?

Eph 3:3
that by revelation there was made known to me the mystery, as I wrote before in brief.

Eph 3:4
And by referring to this, when you read you can understand my insight into the mystery of Christ,

Eph 3:5
which in other generations was not made known to the sons of men, as it has now been revealed to His holy apostles and prophets in the Spirit;

What is this mystery which in other ages was not made known? Paul says that it has now been revealed by the Spirit to Elohim's set-apart apostles and prophets? The next verse tells us exactly what it is.
Eph 3:6
to be specific, that the Gentiles are fellow heirs and fellow members of the body, and fellow partakers of the promise in Christ Jesus through the gospel,

The mystery is that the Gentiles should be fellow heirs of the same body as a part of Israel. Gentiles are no longer strangers of the commonwealth of Israel and are no longer aliens of the covenants of promise.

Eph. 2:12-13 remember that you were at that time separate from Christ, excluded from the commonwealth of Israel, and strangers to the covenants of promise, having no hope and without God in the world. 13But now in Christ Jesus you who formerly were far off have been brought near by the blood of Christ.

Eph. 2:19 So then you are no longer strangers and aliens, but you are fellow citizens with the saints, and are of God’s household,

Eph. 3:6 to be specific, that the Gentiles are fellow heirs and fellow members of the body, and fellow partakers of the promise in Christ Jesus through the gospel,