“Rigid belief systems alone do not define a cult. Far too broad.”
It is not the rigid belief system that is the basis for a cult. It is their total devotion to the leader without question that makes a cult - the leader rules their lives and they let him. That could be five or ten or hundreds or thousands or millions.
Total devotion is a rigid belief system. These terms are not mutually exclusive.
Having total devotion to Jesus does not make you a cult member. Being on a compound with David Koresh makes you a cult member. The difference is group size.