How were the mysteries kept secret since before creation revealed to us?
God, by His grace, released the things that had been kept secret and shared them with us through the Apostle Paul. Jesus did not reveal the mysteries, and Peter did not have them. Paul was not a whit behind the chiefest apostles in authority, so these hidden truths came through his ministry.
Les explains that the secret of God, the Father, and of Christ—in whom are hid all treasures of wisdom and knowledge—had been kept secret. God is now seen fit by His grace to release those things and share them with us through the Apostle Paul. Jesus did not reveal the mysteries, and Peter did not have the mysteries. Paul, referring to the twelve and Jerusalem leaders, says he was not a step behind Peter in authority and was more a minister of Christ than those were.
How that by revelation he made known unto me the mystery; (as I wrote afore in few words,
And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ:
Even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints: