Where are all the treasures of wisdom and knowledge hidden, and why?
All treasures of wisdom and knowledge are hidden in Christ, the secret of God the Father and of Christ, because He is God. That secret was kept hidden but is now released by His grace and shared through the Apostle Paul.
Les explains that the secret of God, the Father, and of Christ is where all the treasures of wisdom and knowledge are hid, and the reason is simply that He is God. That secret had been kept hidden, but now by His grace God has seen fit to release these things and share them through the Apostle Paul. Les also contrasts this with those who want to go only by what Jesus or Peter said, noting that Jesus did not reveal the mysteries and Peter did not have the mysteries, while Paul affirms his apostolic authority and says he was not a whit behind the very chiefest apostles.
That their hearts might be comforted, being knit together in love, and unto all riches of the full assurance of understanding, to the acknowledgement of the mystery of God, and of the Father, and of Christ;
In whom are hid all the treasures of wisdom and knowledge.