What does Ephesians 3:5 mean?

This verse contrasts a mystery, made clear to apostles and prophets in Paul's generation, which had not been explained in the past. The reference to "sons of men" is interesting, used elsewhere only in the Old Testament in relation to people in general (2 Samuel 7:14; Psalm 45:2). Paul contrasts "other generations" with the generation in which he and his readers lived. This was an exciting time, during which Jesus lived on earth and fuller revelation was given to His apostles and prophets.

The emphasis on "by the Spirit" is also important. The church age is marked by a special presence of the Holy Spirit. This began on the Day of Pentecost (Acts 2). The Spirit's indwelling presence in all believers included revealing this particular mystery to the early church's leaders: that Jews and Gentiles are now one in Christ Jesus. This took some time, as church leaders had to meet to discuss this issue, in Jerusalem (Acts 15). Paul and Peter still struggled over this topic, as explained in Galatians. However, the church rapidly came to fully understand that God had given the Spirit to Gentiles in the same manner as He had with Jews. There was no distinction; both were equal in the body of Christ.