"How do you know what is right and wrong if you don't believe in (our) god?"
It is not unusual to hear them ask what stops us from killing and stealing and raping, and it is not even unusual to find people - christian people! - who will claim that if there was no god then nothing would stop them from a frenzy of 'sin'. I find that quite shocking!
Strangely, atheists have no problem knowing the answer to this. If anything, we struggle to understand how the question is in any way meaningful. We don't need to use data (such as the observation that only 0.2% of people in prison in USA are atheists, whereas perhaps 15% of the population are 'out' atheists). We just know that some things are right and some are wrong, and can distinguish between them quite easily.
But let us turn the question the other way round.
"Dear Christians - if you know that you can be completely forgiven for all your sins, what stops you from killing and stealing and raping as much as you want to just now?"
Doesn't that question sound ridiculous when it is that way round? Doesn't it sound offensive?
Many atheists are similarly outraged and offended when you ask it your way too - please remember that.
Incidentally - this is not really intended to be a question for Catholics. I have noticed that this concept has not escaped some of them. Whole 'Catholic countries' cheerfully ignore some of the teachings of their church and perhaps they confess it - or perhaps they don't.