It is not a rare thing in the Old Testament that the surrounding nations are included as participants in the blessings and worship of Yahweh. Even in the promises to Abraham, God says that in him “all the families of the earth shall be blessed.” This promise is not just for the time after the inauguration of the New Covenant. The situation that existed between the Jews and Gentiles is not the same as that which exists today between Christians and non-Christians. The Jews were certainly the people of God, but they were set apart as His priestly people, a kingdom of priests, ministers to the larger Gentile world. When Paul talks about the mystery of the inclusion of the Gentiles, he’s not saying that only now, finally, they can worship Yahweh. By the time of the new testament, there are already believing Gentiles throughout the known world. The mystery is that the distinction, the dividing wall, was being removed. The book of Acts is the plundering of the old covenant. It is not taking Gentiles out of paganism, but rather taking both Jew and Gentile out of the old covenant approach to God – through animals and veils – and weaving them together into one new body that approaches the Father through the Son.
In light of this great work that God was doing, this forming of the church to be the beautiful and radiant image of His light and glory in the world, Paul prays that the Ephesians, along with all the saints, would know the unknowable, the love of Christ which is above all knowledge, and that they would be filled with the infinite, with all the fullness of God.