What does the mystery Paul revealed basically amount to?
The mystery Paul revealed basically amounts to this: Gentiles would become fellow heirs and joint heirs with Christ in the Body of Christ. Everything that belongs to Christ becomes the rightful property of the believer. Israel never had a promise of joint heirs with Christ; this grace was revealed for Gentiles.
Les explains that Ephesians 3.1 introduces Paul as the prisoner of Jesus Christ for the Gentiles and the dispensation of the grace of God given to him. By revelation, Paul made known the mystery: the Gentiles were going to become fellow heirs with Christ. Israel never had a promise that they would be joint heirs with Christ. Les illustrates this in lawyer terms by saying what belongs to one belongs to everybody, so everything that is Christ is now going to be the rightful property of the believer as Gentiles are joint heirs in this Body of Christ. He also notes that Paul was not the last apostle; others followed during the apostolic age, and the mystery was now revealed to holy apostles and prophets.