After talking about the salvation we are receiving, and looking forward to, Peter takes a look back at those who initially wrote about it. The Old Testament prophets wrote under the inspiration of God. But that does not mean they always fully understood the message God gave to them. Peter says that they searched intently to understand the mystery of salvation that they wrote about. But it was kept hidden until it was fulfilled in Christ.
I keep asking that the God of our Lord Jesus Christ, the glorious Father, may give you the Spirit of wisdom and revelation, so that you may know him better. I pray that the eyes of your heart may be enlightened in order that you may know the hope to which he has called you, the riches of his glorious inheritance in his holy people, and his incomparably great power for us who believe. - Ephesians 1:17-19 NIV
All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness, so that the servant of God may be thoroughly equipped for every good work. - 2 Timothy 3:16-17 NIV