You can turn it back on them. Give a little look of surprise and say,
"So are you saying that the Societys teachings are negative?"
Wait for them to answer so that the cognitive dissonance can set it.
After they huff and puff and and make some feeble protest, say,
"How can we teach others if we don't have a full understanding of what the Society teaches both past and present? I don't know about you but I would be embarresed to go out preaching door to door if I didn't even know our own history!"