Why was the mystery hidden in God?

The mystery was hidden in God because He is sovereign, and it is His prerogative to keep things secret. It had been hid in God from the beginning of the world or age—the same God who created all things by Jesus Christ.

Les teaches that the mystery, or secret, was hidden in God from the beginning of the world or age. He explains that this was possible because God is sovereign, and it is His prerogative to keep things secret if He wants to. The mystery was hid in the same God who created all things by Jesus Christ. The context also notes that God had the Gentiles on His mind, and the revelation concerns Gentiles becoming fellow heirs, of the same body, and partakers of promise.

Why was the mystery kept hidden in God instead of being revealed earlier?

The mystery was kept hidden in God because it was His sovereign prerogative to keep things secret as long as He desired. It had been hid in God from the beginning of the world/age, and God later made it known through Paul, that Gentiles should be fellow heirs and of the same body.