Was the mystery of the church kept secret until it was revealed to Paul?
Yes. The mystery of the church was hidden in the mind of God and kept secret until God revealed it to Paul. Scripture shows nothing of the church in the Old Testament or the Gospels; it is strictly a Pauline revelation, the sphere of grace doctrine given during Paul's private training.
Les teaches that God took Saul out of Damascus and, for three years in a private, solitary seminary training, revealed to Paul the mysteries. Those mysteries become the whole sphere of grace doctrine. Les explains that nothing of the church or the body of Christ is found in the Old Testament or the Gospels; it is strictly a Pauline revelation. Paul appeals on that behalf that these revelations of grace doctrine were revealed to him, and that up until that time it had been kept secret and hid in the mind of God. Les also notes that God, as the eternal sovereign Creator, could hide things if He wanted to.