"So are you." (A version of "I know you are but what am I.")
You'll get a WTH look back, to which you reply that the other person left their religion du jour and they're apostates from that religion. Also, dubs are using the books of the bible selected by the Catholic Church back in the day, so the whole jw religion is apostate from Catholicism in a way. Also, dubs split off from 7th Day Adventists, blahbety blah blah blah.
Also, "Yes, that's a word meaning I left a church. Some cults, such as Mormonism, have attached extra meanings to the word, going beyond what is written in the dictionary and bible. Some say it means someone is fundamentally evil. One hallmark of cults is in-group language. Do you know what in-group language is?" And then give the death stare back without breaking eye contact or speaking. Your goal here is awkward silence--hold onto it.