Now to him who is able to strengthen you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery that was kept secret for long ages...
Romans 16:25 ESV

When Paul says, “according to the revelation of the mystery that was kept secret”, he doesn’t mean that God was deliberately hiding it from everyone, and Paul was the first to reveal it. He means that men’s minds were clouded by their idolatry and selfishness so that they blinded themselves to the truths that God passed down through the Patriarchs, delivered through the Prophets, and implanted into all of Creation.