Was there a time when nobody knew this mystery?
Yes. The mystery was hidden in God from the beginning of the world, from Adam, and was not made known in other ages or generations to the sons of men. It stayed veiled until God unveiled and revealed it by revelation.
Les explains that the word "revelation" means an unveiling, something kept under wraps suddenly unveiled. The Lord made known to the apostle the mystery, the secret, by revelation, as written before in a few words. Verse 5 says this mystery in other ages or generations was not made known unto the sons of men, but is now revealed. Verse 9 says it was from the beginning of the world, from Adam, hidden, veiled, and covered up in God, the same God who created everything through Jesus Christ. Les emphasizes that this tells us there was a time when no one knew these things. It also includes that the Gentiles should come in under God's grace.
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: