When we think of a mystery we usually consider it something beyond human understanding.
Biblical mysteries are a little different.
They deal with things beyond unaided human understanding. They are secrets or hidden truths that are only revealed to the mystic, which is one who is initiated. The word mystic was first used by the mystery religions of the Greeks. To them the mystic was one who was initiated into special secrets that were not to be shared with those outside the religion. These secrets had to do with certain rituals performed within the group.