Grace was the Father revealing the mysteries to the apostles and that is what they instructed the perfect with. The mysteries in the old testament were hid until the time of the apostles for them to unveil it. This revealed knowledge wasn’t for everyone, only for the sincere in heart. There was also knowledge that was passed down from the apostles that they didn’t write down as the Hebrews did the same thing. This is called apostolic tradition as referenced in 2 Thessalonians 2:15.
