According to the OED, it is the –

Initiation into mysteries; instruction in arcane knowledge preparatory to such initiation. Often derogatory with suggestion of obscurantism, charlatanism, etc.

I originally came across ‘mystagoguery’ in relation to the presentation of material by planetariums and the fear that they would descend into a morass of science fiction and pure entertainment.  Instead of focusing on education, many thought that planetariums would be forced to play on the mysteries of the universe to bring in an audience.  For more information about this situation, please read my dissertation.