What does it mean that the Church was a mystery kept secret in God's mind?
It means the Church, the Body of Christ, was not revealed in the Gospel accounts but remained hidden in God's mind as part of the mysteries. Because it was a secret, no part could slip out beforehand, so Jesus gave no hint of calling out a Gentile body for Himself.
Les teaches that the Church as we know it, the Body of Christ, is kept secret in the mind of God, and this secret is part of the mysteries. For it to remain a secret, no part of it could be allowed to slip out. Les explains that everything in the Gospel accounts—Matthew, Mark, Luke, and even John—is still predominantly directed to the nation of Israel. He asks listeners to check the Gospel accounts for themselves and see if Jesus ever gives any hint of turning to the Gentiles and calling out a Gentile body for Himself, and he says you will not find it.