What I've read about Greco-Roman pagan mystery religions just includes accounts of the beliefs and points out that practices were kept secret. I haven't been able to find anything that actually explains the purpose served by the secrecy other than a passing speculation that the secrecy helped create a closer bond between members.
I'm also beginning to get the impression that the term 'occult' is something of a misnomer because practices traditionally regarded as occult such as tarot card reading or astrology are not secret at all and 'how to' books or websites can be readily acquired.
However, when occult practices are explicitly deviant, such as shrinking heads, voodoo or casting spells, I still want to ask my question. I suspect, people who are doing evil simply do not want to attract disapproving attention. But what kind of person then would be a definitive authority on this subject?