Paul reveals an age old mystery: How are Jews and Gentiles related to each other through the gospel?
Ephesians 3:6 NIV -
This mystery is that through the gospel the Gentiles are:
1. Heirs together with Israel (see 1:3-14),
2. Members together of one body (see 2:14-16), and
3. Sharers together in the promise in Christ Jesus (see 2:12-13).
I think this mystery is difficult for some on both sides of the Jew/Gentile divide to accept. Difficult to understand, perhaps. But it shouldn't be difficult to accept.