I don't think the answer is simple. The reason being a cult is a specific instance of a radical belief system. If you want something humorous then the audience would need to be introduced to a basic understanding of their ideals.
It seems to me that many belief systems/ideologies are the result of a perceived problem. For this reason although their own tenants are good, the oppositions faults are viewed vehemently.
Consider the major political parties of your country, and insults each may have for the other. Consider the fear differing economic systems invoke in some people. The fear of different races, by racial pride groups. There are also many examples in science fiction too where one group has departed from another by some social, technological, evolutionary... well it's science fiction could be anything... the point is without understanding the differentiating factors between the two groups, insults have little meaning.
Oh, I did think of something! If it's a run of the mill "Kool-Aid cult" then PEPSI DRINKERS! Or maybe "Those who do not partake of the anti-freeze." I think the later would be said solemnly, like you would "Poor misguided people."