“Whereof I am made a minister, according to the dispensation of God which is given to me for you, to fulfil the word of God” (Colossians 1:25).
In Ephesians chapter 3, we find the answer: “ For this cause I Paul, the prisoner of Jesus Christ for you Gentiles,  If ye have heard of the dispensation of the grace of God which is given me to you-ward:  How that by revelation he made known unto me the mystery; (as I wrote afore in few words,  Whereby, when ye read, ye may understand my knowledge in the mystery of Christ)  Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit;  That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:….  And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ:”
While the resurrected, ascended, and glorified Lord Jesus Christ revealed the mystery to Paul, the Holy Spirit used Paul’s preaching and writing to show that secret to everyone else. In our Dispensation of Grace, there is a special body of believers in Christ Jesus, an entity unknown prior to Paul. It is the Church the Body of Christ, “the mystery of Christ” (Ephesians 3:4), or “the fellowship of the mystery” (Ephesians 3:9). The Body of Christ is made up of any Jews and any Gentiles who have simple faith in Jesus Christ’s finished crosswork as sufficient payment for their sins. This body of believing Jews and Gentiles was completely unheard of until God revealed it to Paul. Prior to Acts chapter 9, Jews and Gentiles were separate in God’s eyes. One of the major “household rules” today is God sees no difference between Jew and Gentile. He sees people either in Christ (saved) or in Adam (lost).
Keeping this body of believers separate from the nation Israel is crucial to our understanding of the Holy Scriptures… source