What is the mystery that Paul says was kept secret since the world began in Romans 16:25?
The mystery kept secret since the world began is the whole circle of Paul's doctrines, which God had kept hidden in His mind until He revealed them to the Apostle Paul. This revelation includes the preaching of Jesus Christ and the deeper doctrines later unfolded in Ephesians, received by divine revelation.
According to the transcript, Romans 16:25 speaks of God's power to establish believers according to the gospel and the preaching of Jesus Christ, which is according to the revelation of the mystery. Les explains that this mystery, or secret, had been kept in the mind of God and hidden since the world began. It is not a small point but the whole circle of Paul's doctrines, kept secret until God revealed them to Paul. Les says much of it came out during the three years at Sinai, with deeper revelations appearing in Ephesians, and he suggests the Lord may have poured out these deeper doctrines while Paul was imprisoned in Caesarea waiting to go to Rome. He then directs attention to Ephesians 3.
Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began,
But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith: