Not being able to define a term to someone else’s satisfaction on the spot doesn’t mean you’re talking about nothing rather than something. (It really helps to be able to, of course, but there’s a reason it’s not a convincing argument in practice.)
Still, if they try to define what a cult is (even if they do that later, after the conversation), that alone can help them answer their underlying questions.
You should ask those people what a cult is. They won’t be able to answer, and they may just realize that their question was nonsense to begin with.
Not being able to define a term to someone else’s satisfaction on the spot doesn’t mean you’re talking about nothing rather than something. (It really helps to be able to, of course, but there’s a reason it’s not a convincing argument in practice.)
Still, if they try to define what a cult is (even if they do that later, after the conversation), that alone can help them answer their underlying questions.