Mysteries demand a solution — they have answers that must be revealed — and this is especially true of “the mystery of Christ.” This mystery has been opened up for all the church to understand, for Paul and the other apostles were inspired by the Lord’s grace and through the Holy Spirit to deliver this revelation to the people of God (Eph. 3:4–5). Although this mystery in its fullness is the unveiling of the depth of the person and work of Christ — the gospel — one element of this mystery receives particular emphasis in Paul’s writings. We are speaking, of course, of the mystery that Gentile Christians are put on equal footing with Jewish Christians before the Father.
Lest the readers of Ephesians claim to misunderstand what Paul has already said about the position of the Gentiles in Christ (2:19–22), the apostle tells us once more that Gentile believers are “fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel” (3:6). This indeed is a mystery not understood clearly under the old covenant. It is true, of course, that the prophets looked forward to the day when the Gentile nations would serve Yahweh, the one true God, in faithfulness (Isa. 60:1–3; Micah 4:1–5). Yet the prophets tended to emphasize that the nations would serve Israel in the last days (Isa. 60:10–12), so there was almost no first-century Jewish expectation that the Gentiles would have the same status in the messianic kingdom as the physical descendants of Abraham. This explains why Peter, a Jewish Christian, had trouble believing that his ethnicity meant little in the context of the new covenant. God had to speak directly to this apostle to show that a change was taking place in His dealing with the nations. Even then, Peter fell back into his old beliefs at least once when he refused fellowship with Gentile believers in Galatia (Acts 10; Gal. 2:11–14).
The prophets were not wrong to speak of the Gentiles serving Israel. After all, Jesus is the true Israel (Matt. 2:13–15), so the prophecy that Israel would rule over the nations is fulfilled as Gentile Christians follow Him. Nevertheless, God has chosen to make Gentile believers full heirs of all His covenant promises, and this is a teaching we understand clearly only under the new covenant.