How long was the mystery of Paul's gospel kept secret?

Scripture says Paul's gospel was kept secret in the mind of God since time began, indeed since the world began. It was not made known through the Old Testament prophets, the Abrahamic Covenant, or prior revelation, but was revealed as the mystery concerning the preaching of Jesus Christ.

Les explains from Romans 16:25 that Paul's Gospel is "the preaching of Jesus Christ according to the revelation of the mystery." This mystery was kept secret in the mind of God since time began and since the world began. It was not according to the prophets, the Old Testament, or the Abrahamic Covenant. Very few understand this tremendous verse, but Paul's Gospel is that hidden mystery now revealed.

Romans 16:25

Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began,

When was the mystery kept secret?

According to Romans 16:25, the mystery was kept secret in the mind of God since time began, since the world began. It was not made known through the Old Testament prophets or the Abrahamic Covenant but was hidden until the revelation given to Paul.