OK
I'd say that the mystery is that the Gentiles would be welcomed into the fold of the sheep of God.
This was prophesied in the OT, for example by the Covenant with Abraham....
He would be the father of many nations.
It was completed in the NT when Paul preached to the Gentiles telling them of the god they worshipped which was unknown.
It was not really PREACHED until the NT.
Is it more complicated than this??