The mystery was always a person

In the Bible a mystery is not a puzzle, and not a secret kept for insiders. It is something God kept hidden and has now made known.
Paul says the content of it is not a doctrine, a technique or a hidden knowledge. It is a person, present in his people - and it reaches Gentiles as well as Jews.
Scripture links:
- Colossians 1:27 - "Christ in you, the hope of glory." - The mystery is a person, not information.
You are not waiting to be let in on something. If Christ is in you, you already hold what God kept hidden for ages.