Where was the mystery hidden?
The mystery was hidden in God. According to Scripture, this secret was concealed from the beginning of the world or the age in the sovereign God, the same God who created all things by Jesus Christ. It was His prerogative to keep it secret.
Les explains that the mystery, or secret, was hidden in God from the beginning of the world or the age. In Ephesians 3, Paul writes that he was made a minister to make all men see the fellowship of the mystery, which had been concealed in the sovereign God—the same God who created all things by Jesus Christ. Les emphasizes that it is God's prerogative to keep things secret if He wants to. He also notes that God had the Gentiles on His mind, and that the mystery includes Gentiles becoming fellow heirs, of the same body, and partakers of the promise. In Colossians, Les points out another aspect: Israel would be spiritually blinded for almost two thousand years, while Gentiles come into a privileged position with Israel's God in the body of Christ.
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: