I have faced similar scenarios several times before. Initially I would give them a polite 'fuck off'. I learned that they didn't take this seriously and simply used it as a social cue that the subject was somehow taboo; since I was one of those polite atheists. I was always dismayed, so I developed a new tactic: "Oh, please, do go fuck yourself".
I can't properly convey the manner in which I express it, but it does have a very high damage rate. This phrase must be expressed calmly and with a certain rhythm that communicates that you know something he/she doesn't (which is the truth). The emphasis should be on the 'oh', as if to say idiot. 'Please' should be said in a an almost begging fashion that communicates that the sentiment was fuck all nuts, but I'll give you a chance to redeem yourself. The 'do go fuck yourself' bit conveys the sincere meaning that you take it seriously and that you are focused in your derision so that the recipient takes it personally.
I can't properly convey the manner in which I express it, but it does have a very high damage rate. This phrase must be expressed calmly and with a certain rhythm that communicates that you know something he/she doesn't (which is the truth). The emphasis should be on the 'oh', as if to say idiot. 'Please' should be said in a an almost begging fashion that communicates that the sentiment was fuck all nuts, but I'll give you a chance to redeem yourself. The 'do go fuck yourself' bit conveys the sincere meaning that you take it seriously and that you are focused in your derision so that the recipient takes it personally.