Why was Paul's mystery in 1 Corinthians 15:51 not known until his time?

Les explains that the Body of Christ, what we now know as the Church Age, is strictly a Pauline revelation; it is not found anywhere else except in the letters of Paul. Therefore, Paul is the one in 1 Corinthians 15:51 who says, “Behold, I show you a mystery, which up until his time had been kept secret in the mind of God.” Les teaches that these things were kept secret in the mind of God until He saw fit to reveal it to this Apostle. The mystery concerns that we shall not all sleep or die, but we shall all be changed. This revelation is not associated with the promises given to Israel, but with God’s unique outpouring of grace upon the Gentile masses, whereby they become children of God by faith alone in the Gospel and members of the Body of Christ. Meanwhile, Israel has been in spiritual blindness according to the Old Testament and Paul, blind especially concerning their Messiah, unless the Spirit opens their understanding.