That's not a theory, It's just what the Bible teaches. Look it up.
Eph 3:1-7 (AKJV/PCE)
(3:1) For this cause I Paul, the prisoner of Jesus Christ for you Gentiles, (3:2) If ye have heard of the dispensation of the grace of God which is given me to you-ward: (3:3) How that by revelation he made known unto me the mystery; (as I wrote afore in few words, (3:4) Whereby, when ye read, ye may understand my knowledge in the mystery of Christ) (3:5) 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; (3:6) That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel: (3:7) Whereof I was made a minister, according to the gift of the grace of God given unto me by the effectual working of his power.