Why was the Gentiles being joint heirs with Christ a brand new concept in Scripture?
It was new because Gentile joint-heirship with Christ had been a secret hidden in God from the beginning of the age, not disclosed in Israel’s covenant promises. The concept was something a Jew had never heard, so it was revealed as a mystery for all believers to see.
Les explains from Ephesians 3:8-9 that Paul was given grace to preach among the Gentiles, and this was a whole new concept in Scripture. Earlier in the chapter, Paul says the Gentiles are going to be joint heirs with Christ. That was something a Jew had never even heard of in their covenant promises. But Gentile believers have this prospect: when they go into the eternal abode, they will be joint heirs with Christ. In verse 9, this mystery is to be seen by all believers, not just the spiritual, pastors, or theologians. The fellowship of the mystery had been hid in God, who created all things by Jesus Christ, from the beginning of the age. Fellowship means a meeting of the minds, like-minded thinking about things.
And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ: