Mystery means something Hard to explain and understand because of not natural.
     Salvation remains mysterious among men from the beginning of the humanity.  That is why we see so many Myth or Rumor about Salvation, Heaven and Eternal Life. 

     But Good News came to this world when Redeemer Himself took birth to redeem His people from Sin (Matthew 1:21). Jesus the Redeemer gave Himself on Cross for our Sins and Resurrect on the 3rd day to give us eternal life. 

     Jesus disclose the mystery of Salvation now he who believe in Him, will be Saved (Act 16:31). Happy Christmas.