In this passage, Paul maps out his mission and authority to preach Jesus Christ to the Gentiles even though He did not know Him in the flesh. Paul only knew the resurrected Christ (1 Cor. 15:9). Paul’s mission is to “make the mystery known” (v. 9) by way of and through the Church (v. 10).
The fact that Gentiles were to be saved was not a mystery (Rom. 9:24-33, 10:19-21). The mystery “hidden in God” was the bringing together of the Jew and Gentile to form a wholly new thing “the Church, which is His [Christ’s] body,” formed by the baptism with the Holy Spirit (1 Cor. 12:12-13). In His Church, God does not see any distinction between Jew and Gentile (Eph. 2:4-15, Col. 3:10-11).
The revelation of this “mystery” was foretold but not explained by Christ (Matt. 16:18). Those details, concerning doctrine, position, walk and the destiny of the Church, were left Paul and fellow “apostles and prophets” by the Spirit (v. 5). These details have been committed to (Phil. 1:27, 2 Th. 2:15, Ti. 1:9, Jude 3) by Christians through history in the face of decaying societal morality.