Eph 3:6 This mystery is that the Gentiles are fellow heirs (a), members of the same body (b), and partakers of the promise in Christ Jesus through the gospel (c).
The third point in this verse is that the Gentiles are partakers of His promise in Christ through the gospel. The gospel, good news, is the great instrument and means by which God works faith in us through Christ. This was the great truth revealed to the apostles, namely, that God would call the Gentiles to salvation by faith in Christ without the works of the law. They did not work for it to attain it. God worked in them both to will and to do for His own good pleasure (Phil 2:13).
This revelation the apostle speaks of, we now understand and receive #1. The promise of the Holy Spirit, #2. The promise of His gifts and graces, #3. The promise of the eternal life and joy, as the gift of grace through Christ. All these promises are of and in Christ for our justification, sanctification, growth, and service.
In summary, because of Christ, the Gentiles who were far off have now been brought near and made fellow-heirs. The wall of separation is now broken down, Jews and Gentiles are equally received, heirs alike of the gospel, fellow-members of the church, fellow-partakers of the gospel promises. If this is not the ground for unity what else is? This is why I believe the answer is not out there, the answer is in Christ. This is not a system issue, this is a heart issue.What God has accomplished through Christ in dealing with the heart, no system of thoughts in the world can reconcile the divide apart from Christ.