أنجليزي إلى عربي العلوم الاجتماعية الدين

I don't think there's an easy one-word answer, because طائفة and مذهب don't carry the same (derisive) connotations as 'cult' has in English.

A cult, according to Wikipedia, "usually refers to a new religious movement or other group whose beliefs or practices are considered abnormal or bizarre."

Cults are often associated with highly charismatic but totalitarian leaders as well. Jonestown is a classic example of a cult from recent American history:
http://www.culteducation.com/jonestown.html

Any ideas for how to best precisely convey this meaning in Arabic?