Hi there,
It seems like you are in a hard spot right now. I would like sit them down on a day they are in a pretty good mood and tell them that you are atheist.. Explaining to them why you are and explain that you don't want to be part of something you don't want to be believe in. If you don't want to be so up front with it you can try writing them a note? Or you can get someone you trust (sibling, friend,etc.) Tell them for you and you can explain later.
Just be up front with it. I don't think there is a way were you can say it and have them react. And use a good tone in your voice.. Is the best way to ease it down a little.. They may be shocked or something.. I have no clue on how your parents will react to this.. It depends on how they are as person!
I hope I helped a little! Feel free to
PM/
VM me anytime!