Was the mystery hidden in the Old Testament?
The mystery was present in the Old Testament only in latent form, not openly revealed. God had it in His mind, but Scripture says it was kept secret and no one previously had any concept of it. It was made known later by revelation to Paul for the Gentiles.
Les teaches that the mystery was revealed to Paul by revelation from the ascended Lord. It was that revealed body of truth that no one ever before had any concept of. Yet it was back in the Old Testament in latent form. Now that we understand it, we can go back to the Old Testament and see that God had it on His mind, but He had not revealed it as it was kept secret. Paul makes it plain that all of this was revealed to him, and he then passed it out to the whole world.