What distinguishes a religion from a cult?

What distinguishes a religion from a cult?  I don’t remember how the subject came up yesterday, but my friend John’s immediate answer was “The number of followers.”  The six of us talked around the topic all the way back to the office after lunch, and that was still the most satisfying answer, as far as I’m concerned.  Merriam-Webster‘s definition of the word cult isn’t much help, and the Defining Terms article from the Deception in the Church page, although interesting, just adds more fuel to the fire.  Popular usage of the term cult is all about what’s considered mainstream.

No, there really isn’t a point to all this pondering, beyond noting that one person’s (or group’s) cult may be another’s religion.  Definitions are such slippery things sometimes.