What is a mystery cult? Is it like functional programming?

One of the three major categories of Hellenistic religions. The defining aspect was that their religious beliefs and rites were kept secret and only revealed to initiates. For obvious reasons we don't know a lot about them.

My vague understanding is that the idea that Christianity was significantly derived from local mystery cults was once taken seriously, but it's completely fallen out of favor and everything new we've discovered about the mystery cults has made the connections ever more tenuous.