Was the mystery Paul revealed made known in past generations or the Old Testament?

Les explains that Ephesians 3:5 says the mystery of Christ was not made known in other ages, in other past generations, which takes us all the way back to the Old Testament. These things Paul was now revealing had never been made known before, and it was not made known to the sons of men as it is now revealed. Les stresses that once it is revealed, it belongs to us, which means we better believe it. He notes this segment of the mystery concerns the Gentiles being fellow heirs, of the same body, and partakers of His promise in Christ—not by Judaism, not by the Mosaic law, but by Christ and His finished work. He contrasts this with the Old Testament, asking whether anyone ever read there that Gentiles would become one with Christ or with the God of Abraham.