The mystery of the gospel is that it was opened up to all.
When were YOU ever under the law?
That is clearly not the mystery in Eph 3. The grammar does not support that and the OT is full of God doing things with the Gentiles in the background.
The OT is actually about the coming mission of Messiah. It is not about setting up a Zionist paradise then or future.
There was no mystery at all that the Gentiles were to be reached. The Pharisees had missionaries because they knew this, and Jesus blasted them on their message, and type of impact.
The outreach to the nations was "known for ages" says James in Acts 15, quoting Amos 9.
The grammar of Eph 3:5-6 is saying that the mystery was that the sharing of Israel's inheritance and citizenship--the one that is by faith--is
through the Gospel. But you won't read Greek grammar commentaries, nor study Greek, so you will never know. The weight of those two lines falls on 'dia tou euangelliou' because this is to contrast with the Law, as Paul so often does. It is true through the Gospel, not in the Law.
The Gospel of Christ was preached in advance (Abraham saw my day and rejoiced) says Gal 3 referring to the lines about 'in your Seed all the nations of earth will be blessed.' Israel/Judaism replaced this message with the law, Gal 3:17. That is the replacement theology that matters.
The message or goal or destiny for Israel never was a Zionist kingdom. It always was a kingdom that was not the kind found on this earth.