Why can God now reveal the mystery that was hidden before creation?
God can now reveal the mystery because the price of redemption has been paid, the work of the cross is done, and the power of His resurrection is accomplished. What was kept secret since before anything was created can now be poured out on the human race as the whole package of revealed truth through the Apostle Paul.
Les teaches that the mystery was kept totally secret in the mind of God since before creation, which is why Jesus and Peter could not preach it. But because the price of redemption was accomplished—the work of the cross done and the power of His resurrection—God can now pour out on the human race the whole package of revealed truth from the pen of the Apostle Paul. That package contains all the truth the Church needs today. This does not mean throwing away the rest of the Bible; all the Bible works for its common end. Les suggests that if Christendom spent 90% of its time in Paul's epistles and 10% in the rest of the Bible, things would begin to happen, but it is 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,