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.

Les teaches that Romans 16:25 describes the mystery as kept secret in the mind of God since time began—since the world began. He emphasizes that Paul's Gospel is the preaching of Jesus Christ according to the revelation of the mystery, not according to the prophets, the Old Testament, or the Abrahamic Covenant. Les notes very few people comprehend this verse, and most don't even read it. The Bible says the mystery was kept secret since the world began.

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,

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.