Paul reminds his readers that he had touched on this subject back in chapter 1. Remember, a "mystery" in the Bible refers to information not yet revealed to others. What Paul has heard from the Spirit has to do with Christ. He makes special mention that the Lord did not reveal these things to those in Old Testament times. But God has made these plans known to the apostles and prophets of Paul's day. And this should cause us to want to learn what this mystery is about and how it will affect us since we belong to Christ.
Paul was privileged to watch the unfolding of God's plan in the very events of his day. And he was able to help believers get an understanding of what they were seeing happen in before their eyes.