Yes, they were reconciled to God but what does the passage say they were separated from? What does the passage say separated them? Once reconciled, what were they joined to?
Separated from Israel.
Eph. 2:12 that at that time you were without Christ, being aliens from the commonwealth of Israel and strangers from the covenants of promise, having no hope and without God in the world.
Separated by the law of Moses.
14 For He Himself is our peace, who has made both one, and has broken down the middle wall of separation, 15 having abolished in His flesh the enmity, that is, the law of commandments contained in ordinances,
Joined to Israel
....so as to create in Himself one new man from the two, thus making peace,16 and that He might reconcile them both to God in one body through the cross, thereby putting to death the enmity. 17 And He came and preached peace to you who were afar off and to those who were near. 18 For through Him we both have access by one Spirit to the Father. 19 Now, therefore, you are no longer strangers and foreigners, but fellow citizens with the saints and members of the household of God,
After this is when Paul tells them the mystery. If I understand your view of the mystery, it’s wrong. The mystery is that the Gentiles would be added to Israel not a separate group.
Eph. 2:6 that the Gentiles should be fellow heirs, of the same body,
Rom. 11:17 And if some of the branches were broken off, and you, being a wild olive tree, were grafted in among them, and with them became a partaker of the root and fatness of the olive tree,
Why would Paul testify to the gospel of grace by preaching the kingdom of God?
Acts 20:24 But none of these things move me; nor do I count my life dear to myself, so that I may finish my race with joy, and the ministry which I received from the Lord Jesus, to testify to the gospel of the grace of God. 25 “And indeed, now I know that you all, among whom I have gone preaching the kingdom of God, will see my face no more.
