28 Be it known therefore unto you, that the salvation of God is sent unto the Gentiles, and that they will hear it.
The dispensational boundary which occured in Acts 28:28-31 is when the Apostle Paul declares to the Jewish leadership that the final opportunity for their acceptance of Jesus as Christ is over. From that point on the message for the plan of salvation is now authorized to be sent from the Gentiles, apart from Israel. Prior to this point in scripture, salvation was always either through or with the Jew. At this point the nation of Israel has been put on hold but will be still receive the promises and blessings made to them through Abraham in the future (when Elijah restores the revealed plan of God). Consequently, for the first time in scripture, Paul declares that whosoever will accept Christ and this calling, they become a member of the body of Christ where Christ is the head. This calling now has its own blessings and inheritance separate from Israel. To explain this “hidden plan” that is now revealed, Paul then begins to write the final 7 books in scripture, chronologically (Ephesians, Philippians, Colossians, Titus, Philemon, 1 Timothy, and 2nd Timothy.) The reason we describe this calling as the hidden purpose is that it was hidden in God, before the foundation of the world and not revealed until Paul was told to do so in Ephesians 3:9. The primary name given to us by the Apostle Paul for the special body of truth during this timeframe is called “The Mystery” (Ephesians 3:9, Colossians 1:25-26) and it is our calling for today.