A mystery is something secret, hidden and not perceived by ordinary means.  In the book of Daniel a significant mystery is revealed through divine revelation (Daniel 2); Paul speaks of a mystery of God in Romans 11 and again in Ephesians 3.  In speaking of the mystery of resurrection (1 Corinthians 15) Paul says that believers “will be changed, in a moment, in the twinkling of an eye, at the last trumpet.”