Why couldn't Jesus and Peter preach the mystery that Paul later revealed?
The mystery was kept secret in God's mind since before creation, so it had not yet been revealed during Jesus' and Peter's ministries. Only after the work of the cross and the power of His resurrection could God unveil this package of truth through the Apostle Paul.
Les explains that God kept the mystery, or secret, hidden since before anything was created. That is why Jesus could not preach it and Peter could not preach it—it was still kept totally secret in the mind of God. But because the price of redemption was accomplished, the work of the cross is done, and because of the power of His resurrection, God can now pour out on the human race this whole package of revealed truth. That package comes from the pen of the Apostle Paul, and in it we have all the truth the Church needs today. Les adds that this does not mean throwing away the rest of the Bible, because all the Bible works toward its common end. Yet if Christendom spent 90% of its time in Paul's epistles and 10% in the rest of the Bible, things would begin to happen, instead of doing it the other way around.
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,
But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith: