Ephesians 3:3-6 NKJV
how that by revelation He made known to me the mystery (as I have briefly written already, by which, when you read, you may understand my knowledge in the mystery of Christ), which in other ages was not made known to the sons of men, as it has now been revealed by the Spirit to His holy apostles and prophets: that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ through the gospel,
Paul and others received revelation of the mystery of the Gentiles who received the gospel becoming fellow heirs with the Jews, partaking of the Abrahamic covenant through Christ. This revelation was in the Old Testament but was hidden from those who read it. The original apostles were shown this mystery by the Holy Spirit. There can be things hidden in plain sight because the understanding has to be received from the unseen realm. When we receive revelation from the unseen realm, it does not contradict the Word, it reveals it. We must go beyond the routine of religion and seek the Lord with our hearts on a more desperate level, or we will remain in shallow water. We may have read something a dozen times before, but as we press in for more from God, understanding from the unseen realm will come off the pages and change us.