Where was the mystery kept hidden before it was revealed?

The mystery was kept hidden in God from the beginning of the world, or the ages, or Adam. Scripture says it was hid in God, the same God who created all things by Jesus Christ, and remained a secret throughout the Old Testament until it was revealed.

Les explains that Ephesians 3:9 gives the answer: the mystery, or secret, was hidden in God. It had been concealed from the beginning of the world, the ages, or Adam, and was kept secret all the way up through the Old Testament. During that time, not a word was given about God going to Gentiles with the Gospel of the Grace of God. Les notes that this is why it had to be revealed. He also points to Galatians 3:14, where salvation now came to the Gentiles through Jesus Christ. Earlier, he connected this to the Law: before the Faith Way came, people were kept under the Law, shut up until the Faith should afterwards be revealed. So the mystery was not hidden in Scripture or in a place on earth, but in God Himself, the same God who created all things by Jesus Christ.

Ephesians 3:9

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:

Where was the mystery kept hidden since the beginning of the world?

Paul says the mystery was kept hid from the beginning of the world; it has been hid in God, the same God who created all things by Jesus Christ. This hiding was God's own prerogative, and He did it; the secret was not revealed from the world's beginning.