Webster's Dictionary defines a cult as:
1. A formal religious veneration
2. A system of religious beliefs and rituals also its body of adherents
3. A religion regarded as 'unorthodox or spurious'
4. A system for the cure of disease based on dogma set forth by its promulgator
5. a: A great devotion to a person, idea, thing; esp.: such devotion regarded as a literary or intellectual fad, b: A usually small circle of persons united by devotion or allegiance to an artistic or intellectual movement or figure.
Obviously, this definition could include everything from stamp collectors to diehard Dylan fans. However, the most important concern today is not simply who might be somewhat "cultic" in their devotion, but what groups might represent potential problems regarding personal or public safety. That is, groups that are potentially unsafe and/or destructive.
Note: Recently, someone posted on my site that I encourage the killing of "Moslems". Never (Mathew 5.44)! Rather, I encourage the conversion of Muslims to Jesus Christ alone (John 1.1, 14; 3.16; 14.6). The accuser continued in his post to issue a veiled threat to my safety. I would hasten to say that such a threat falls under the category of cultish behavior. May the Prince of Peace forgive this perpetrator and deliver him from the bondage of sin and death (Romans 6.23; 10.9).