Was the mystery of the Gentiles revealed during Jesus' earthly ministry?
No. The mystery concerning Gentiles was not revealed during Jesus' earthly ministry. It was kept secret in the mind of God until revealed to Paul, who proclaimed what had never been revealed before. Jesus spoke only of Himself and Israel, not a joint heirship with Gentiles.
Les explains that the secret fellowship was kept secret and was not yet revealed during Christ's earthly ministry. Jesus never indicated this kind of consort between Himself and Gentiles; He only talked about Himself and Israel. The secret remained in the mind of God until it was revealed to Paul. Now it comes out to the whole Gentile world, showing the glorious prospect of joint heirship with Jesus Christ. Paul is proclaiming something never revealed before, as also referenced in Ephesians 3:9 and Ephesians 6:19-20.
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: