OK...that is also why it would not be a mystery!
What is mystery (to Judaism) in Eph 3:6 is that the sharing comes through the Gospel. The sharing of inheritance, membership. Not through the Law.
Please remember: most of the NT is spoken to Judaism as it was at that time. That's why Paul was divinely arrested by God; because he was the spox at the time. And as you may notice: there is no small friction with Judaism.
The redemption in the Gospel is no mystery.
The fact that it would go to the nations is no mystery.
That leaves only that the things shared, the co-inheritance, was to come through the Gospel, not the Law.