Noun
A mystery is something that is difficult or impossible to understand or explain.

A mystery is a spiritual secrete or revelation revealed to someone, but hidden to those to whom it is not given.

To encode or to conceal information:

A secrete that enables a person, group of people to exploit and benefit from its spiritual ideas or revelation, while to those to whom its not given may not even know that the mystery exists.

Those to whom it is not given, may be blind, lacking comprehesion or understanding towards the implications of that mystery.

Isaiah 29:11-12 defines what a mystery is:

11 The whole vision has become to you like the words of a book that is sealed, which men deliver to one who is literate, saying, “Read this, please.”

And he says, “I cannot, for it is sealed.”

12 Then the book is delivered to one who is illiterate, saying, “Read this, please.”

And he says, “I am not literate.”

To speak in parables which are are revealed to those who are intimately related to the oracle, and to those who are not closely related may not even understand what is being said, done or transpired.

Teacher of the hidden mysteries:

To his close students or intimate partner, he reveals his mysteries, (he takes time to teach the deeper meaning according to each one’s capacity, ephah or omer),

but to those who are not close to him, he seems to speak in encoded parables.

Mysteries can take on different forms;

There are mysterious actions, movements, speech, gestures, subliminal messages, ideas, intimate relationship, spiritual worlds, platonic love, spiritual environments, teachings etc,

Which are revealed to the one it’s given and hidden to the one’s it is not given.

explain.
“the mysteries of outer space”
synonyms:

puzzle, enigma, conundrum, riddle, secret, unsolved problem, problem, question, question mark, closed book; informalposer
“his death remains a mystery”
2.
A novel, play, or film dealing with a puzzling crime, subliminal messages especially a murder.