Why couldn't Jesus and Peter preach the mystery?
The mystery remained hidden in God's mind from before creation, so Jesus and Peter could not preach it. It was kept totally secret until the price of redemption was paid and the work of the cross was done; then God revealed it through the Apostle Paul.
Les explains that God kept the mystery secret since before anything was created, so Jesus and Peter could not preach it because it was still totally hidden in the mind of God. Only after the price of redemption was accomplished, the work of the cross was done, and because of the power of His resurrection, could God pour out this whole package of revealed truth. That package comes from the pen of the Apostle Paul, and Les says it contains all the truth the Church needs today. Les also notes that this does not mean throwing away the rest of the Bible, but Paul's epistles should have priority.
Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began,