Why did God reveal the church body of truth to Paul rather than earlier?
God revealed the church body of truth to Paul because it was a secret held in the mind of God, and its contents were totally different from anything previously revealed. It was uniquely a church phenomenon, unknown to Israel and the Gentile pagan world, so God unveiled it later, at Acts chapter 9, as a fork in the road.
Les explains that the body of truth given to Paul included doctrines such as the rapture of the church and justification by simple faith in the gospel, which are uniquely church phenomena. Israel knew nothing of such things, and the Gentile pagan world certainly did not know them either. Because all these items were so totally different from anything that had ever been revealed before, they were a secret held in the mind of God until He revealed them to this apostle. At Acts chapter 9, God said He was going to do something different—just like a fork in the road after the Jew-only program since Abraham. The Jerusalem leadership also saw that the gospel of the uncircumcision, meaning Gentiles, was committed to Paul, while the gospel of the circumcision, or the Jew, was committed to Peter.