25 Now to Him who is able to establish you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery kept secret since the world began
according to [standard] my gospel
The word “my” indicates that Paul fully embraced the gospel as his own. The gospel was his standard or norm for doing what he did. God chose Paul alone to reveal the church (Eph 3:1f)—that the gospel would be for both Jew and Gentile. The gospel was the unveiling of the mystery of the economy of the church. Many economies had passed before God revealed the church to Paul, the gospel that includes both Jew and Gentile.
and [that is to say] the preaching [the thing preached] of Jesus Christ,
God provided salvation through Christ. The preaching of Jesus Christ was Paul’s norm for ministry.
according to the revelation of the mystery
It is important to understand that the word “mystery” in the Greek New Testament is not the same as the English use of the word today. Modern English for the word conveys the idea of something spooky. The Greek word does not mean something we cannot figure out or explain. Mystery is not something incomprehensible or obscure. However, this term carries the idea of something hidden in the past and now revealed. The most important passage on this is Ephesians 3:3-5. The idea is that Gentiles are fellow-heirs and fellow-members of the church with Jews.
Paul used the word “mystery” in Romans 11:25, where it referred to the temporary hardening of the Jews until the full number of Gentiles had come in. The idea is that a new time of the Gentiles had come; the Jews’ time was over for a while.
Eph 3: 3 how that by revelation He made known to me the mystery (as I have briefly written already, 4 by which, when you read, you may understand my knowledge in the mystery of Christ), 5 which in other ages was not made known to the sons of men, as it has now been revealed by the Spirit to His holy apostles and prophets: 6 that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ through the gospel,
Co 1: 26 the mystery which has been hidden from ages and from generations, but now has been revealed to His saints. 27 To them God willed to make known what are the riches of the glory of this mystery among the Gentiles: which is Christ in you, the hope of glory.
kept secret since the world began
God hid from believers in the Old Testament the idea of the church but revealed it in the New. He concealed the equal status of Jew and Gentile from Old Testament believers. They knew that the Messiah would come, but they did not know that God would set aside Israel as a nation and turn to the church, in which both Jew and Gentile would hold equal status.
The perfect tense of “kept secret” in the Greek means that the “secret” has been kept from a point in the past with the result going on until the present. God revealed it in the time of Paul.
The church began on the day of Pentecost.
God revealed only the broad panorama of His economy to the Old Testament prophets, but now He revealed the specifics to the apostle Paul. The formation of Jews and Gentiles into an entity we call the church was the mystery.