Ephesians 3:6 This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel.
When ever you run into a gray area or a verse that’s seems a little odd, or even a verse that is mysterious, the best rule to follow is “Let scripture explain scripture”. God is not the author of confusion. 1 Corinthians 14:33 For God is not a God of confusion but of peace. As in all the churches of the saints. First seek wisdom from God, next you must dig deep to find the purest oil. If a verse is difficult it is meant to be, it is God’s way of getting us to dig a little deeper. But Paul reveals the mystery so we need not to dig but just digest it. The mystery being that God has made the Gentiles joint heirs to Abrahams promise. It was by faith back then and it is by faith today. “With the Jews, of all the blessings of grace, of lasting salvation, and of the eternal, incorruptible, and never fading inheritance in heaven; that they should be heirs of God, and joint heirs with Christ Jesus, and inherit all things: and heirs they are by being Christ’s, and on account of their special adoption through him, and their justification by His righteousness; and which appears by their regeneration to a lively hope of the inheritance reserved for them in heaven”. Gill
All this being brought under one Head, Jews and Gentile in one Church, both partakes if the promises in Christ. How can anyone believe in “dispensationalism”? God, being their covenant God, which is the great promise of the covenant; of, his gifts and graces, called the promise of the Spirit, thus making them eternal; is the gift of grace through Christ. Now all these promises, and all others, are in Christ, Amen. Safe and secure; and it is being in Christ that any can partake of them; and because this mystery now revealed at by Gospel, as these Gentiles were called. The Gospel is a declaration of what God has promised in covenant to his people through Abraham, this was carried by Gods grace to the Gentiles, and was made effectual to their participation of the things contained in it.