I tend to just ask them (all religions) why they believe in God. I have not yet heard one reasonable answer and none of them have told me the truth of "my Mum and Dad told me to".
If you can't be certain of disbelief in a story as ridiculous as God, with absolutely no evidence to support the completely wild religious claims, what can you be certain about?
If i told you i could fly for instance. I can't prove it, but you can't disprove it. So can you not say, for certain...