Paul is saying, I am going to give you a mystery. The word mystery here means something hidden that is now revealed. Hidden for generations. In the Old Testament they didn’t understand. In the Old Covenant they didn’t understand what he is ready to say. This is a new thing. To every person who surrenders to Christ, they understand, it becomes apparent, it becomes reasonable, completely believable. To them God willed to make known what are the riches of the glory of this mystery among the Gentiles: Gentiles? The Jews understood that they were God’s people but they could’t understand that there would be any way the Gentiles could get in. which is Christ in you, the messiah, they were expecting the messiah, there are over 300 prophecies about the messiah. Born in Bethlehem, Enter the city riding on a donkey, betrayed for 30 pieces of silver. All these specific prophecies that were fulfilled in Jesus, but they missed Him, they missed that Gentiles would understand that.
There are two parts to what they missed. One, That gentiles would be fellow airs, - that was inconceivable concept to Jews at the time. Secondly the bigger mystery, that the Messiah would take up residence in you. That God would live in our hearts. This is the fulfillment of a long promised covenant, the new covenant. That is what is being described here.