Eph 3:4-6
"When you read this, you can perceive my insight into the mystery of Christ, which was not made known to the sons of men in other generations as it has now been revealed to his holy apostles and prophets by the Spirit: that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel."
The mysterion of Christ: not that the Gentiles would be saved (this was already announced in the OT), but that they would be fellow heirs, fellow members, fellow partakers — equal to the Jews in Christ. This radical equality was the scandal of the Gospel to first-century Jews.