Not an unknowable secret but a truth formerly hidden and now disclosed — specifically the plan of God revealed in the gospel. Paul uses musterion for the inclusion of Gentiles in the body of Christ (Eph 3:3-6).
The NT transforms the Greek word for pagan mystery-cult secrets into a word for the open gospel. God's mystery is now published, not hoarded — proclaimed to all nations (Rom 16:25-26).
Paul's mystery = Christ + Gentiles together = one body. This was hidden in the OT's shadow but is now proclaimed. The preaching of the mystery is itself eschatological — it heralds the age of fulfillment.