What was the mystery that God kept hidden until He revealed it to Paul?
The mystery is a secret (Greek mysterion) God kept in His own mind until He revealed it to the Apostle Paul. It concerns Gentile believers, but this excerpt does not specify its content beyond that it is a secret disclosed to Paul.
Les explains that the word 'mystery' is one Paul uses often, and in the Greek it is mysterion, meaning secret. A secret is a mystery, and a mystery is a secret. Paul does not want Gentile believers to be ignorant of this secret that God had kept in His own mind until He revealed it to the Apostle Paul. Les stresses this word because it has been a while since he emphasized it, but the context stops before detailing the specific content of the mystery.