Well, here's what I finally came up with:
What is the Mystery?
One of the key themes of the letter we call Ephesians is the unity of the church, particularly as it relates to Jews and non-Jews, or Gentiles. Both Jew and Gentile belong to the same body and have the same Spirit (4:4). They have the same God and Father, the same Lord, faith, and baptism (4:5-6). Ephesians highlights the social unity between Jew and Gentile more than any other book in the New Testament.
Ephesians also reflects how unexpected this development must have been for the early Christians. We have hints of the mysteriousness of this new phase in God’s relationship with the world in earlier letters of Paul. Romans 11 talks about the mystery in slightly different terms. In Romans 11:25 Paul is dealing with the strange situation of his day in which as many or, probably, more Gentiles had come to believe that Jesus was the Lord of the world than Jews.
How strange, Paul recognizes, that fewer among God’s own people believed in their messiah than those outside! Here is the mystery, Paul says, “Israel has experienced a hardening in part until the full number of the Gentiles has come in” (Rom. 11:25). Why it happened this way we do not know. Paul can only exclaim, “Oh, the depth of the riches of the wisdom and knowledge of God! How unsearchable his judgments” (11:33).
By the time of Colossians, the mystery is discussed in more generic terms. Addressing a Gentile audience, Colossians says that the mystery is “Christ in you, the hope of glory” (Col. 1:27). Glory is something Paul taught was the destiny of all believers. As Hebrews tells us more explicitly, God had created humanity to be crowned with glory and honor over the creation (Heb. 2:7). Unfortunately, all have sinned and are lacking the glory of God (Rom. 3:23). At present we do not yet see humanity with this glory (Heb. 2:8). So Jesus came to lead many sons to glory (Heb. 2:10). This “hope of glory” in Colossians is now something that Gentiles as well as Jews can hope for because of Christ.
Ephesians finally expresses this mystery even more straightforwardly than Colossians. The mystery is plainly stated in 3:6 as being the fact “that through the gospel the Gentiles are heirs together with Israel, members together of one body, and sharers together in the promise in Christ Jesus.” This is the “mystery of Christ” (3:4), a mystery that “through his blood” the Gentiles have been brought near (2:13). “In his flesh,” Christ abolished the Law, the dividing wall of hostility that had separated Gentile from Jew, and the two were now reconciled “through the cross” (2:16).
Ephesians uses language here that is different and starker than Romans. For example, Romans 3:31 emphasizes that justification by faith—being considered right with God because of our trust in what he has done for us through Christ—does not “nullify the law.” Paul uses the word law in different ways that can be confusing, but the later distinction Christians made between the “moral” parts of the Jewish Law (like the Ten Commandments) and the “Jew-specific” parts (like circumcision or not eating pork) gets at the gist of what was not nullified (the “moral” parts) and what was abolished, at least in terms of Gentiles (namely, the Jew-specific parts).
That God would do so was a mystery. The Old Testament assumes throughout that God has a special relationship with Israel and that Israel’s blessing was connected to its keeping of the particulars of the Jewish Law, God’s covenant with Israel (cf. Deut. 28). Provision is made for the “alien” in the land (e.g., Deut. 10:19), but the Old Testament as a whole assumes that those outside Israel worship other gods and are almost always God’s enemies destined for judgment. It may be said to Abraham that all nations on earth would be blessed through him (e.g., Gen. 22:18). But prior to Christ that blessing was understood to come through Israel, not directly to the Gentiles the same as to Israel!
A New Revelation
A feature of Ephesians and Colossians (as well as of Romans 16:25, which probably was not part of the original copy of Romans, but nevertheless was added to many copies of Romans very early on) is a sense that this mystery of the equal inclusion of Gentiles alongside Jews “was not made known to men in other generations as it has now been revealed by the Spirit to God’s holy apostles and prophets” (Eph. 3:5). Colossians 1:26 says that this mystery, “has been kept hidden for ages and generations, but is now disclosed to the saints.” In a similar vein, the later ending of Romans says that this mystery was “hidden for long ages past, but now revealed and made known through the prophetic writings by the command of the eternal God, so that all nations might believe and obey him” (Rom. 16:25-26).
These passages reflect how unexpected this development must have been to the earliest Christians, including Paul. Indeed, we remember that before Christ appeared directly to Paul, he thought the very idea that the messiah would die on a cross was foolish, probably fighting words! He was a Pharisee, a group that emphasized separating yourself to God by living a particularly pure life (by the standards of the Jewish Law). What an unexpected turn around for him, now to think that God would include the Gentiles apart from the Law through the reconciling blood of Christ, Gentiles who almost by definition were unclean according to the laws of Leviticus! But it was Jesus who set this trajectory by trying to redeem sinners within Israel like prostitutes and tax collectors. To see Christ’s blood potentially redeeming the whole world, even non-Jews, thus fit the spirit of Jesus’ mission while he was on earth.
The Consensus of the Saints
We have the great benefit of hindsight on these sorts of things. Paul’s writings and Acts give us hints that the full inclusion of the Gentiles was not immediately obvious to everyone in the early church. Arguments along these lines dominate Paul’s letters to the Galatians and Romans and we find hints in Acts 15 and 21 of how live these issues were even within a decade of Paul’s death. But Ephesians and Colossians do not blink to tell us that the apostles and prophets had come to recognize this mystery (Eph. 3:5). Prophets here does not likely refer to the Old Testament prophets but to prophets within the early church. The implication would seem to be that, whatever struggles the earliest Christians might have had in coming to accept this mystery, by the time of the apostles’ deaths and the passing of the first generation, it was agreed that God had done something very unexpected from a Jewish standpoint. Through the atoning death of Jesus Christ, all people now had equal access to God, whether Jew or Gentile.