A ‘mystery’ is more than a literary genre.

Mysteries

The common man watches mysteries in movies or television shows; he reads books in the mystery genre.  These are typically crime stories!  He might even use the word to express an opinion about someone: “it is a mystery how politicians can be so wrong!”.  In this sense, a mystery is an uncertainly that can be understood with diligent investigation. 

A religious man appreciates the additional meaning that a mystery is a description of a belief that is divinely revealed; something that the eye of human reason cannot see alone. 

 The educated man knows as well that at the time of operative masons creating the great cathedrals that a skilled tradesman or member of a guild would refer to his craft, his work, his occupation, as being his ‘mystery’; it is his occupation and how he conducted himself. 

The applicant at the door of the Lodge might think he will be admitted and learn some secret knowledge and share in the mystery of Freemasonry.  The initiated Mason begins to appreciate the deeper meanings of the mysteries, or Work, that he does in Lodge; and why he takes those actions in a just, perfect, and regular Lodge.

It must also be noted that the Oxford English Dictionary includes mention that ‘mysteries’ are “a secret of Freemasonry” and this use entered the language in the mid-1700’s.