My sensitive parents are Christian, I'm not, they ume I am...help?

This is more of an etiquette question than religious, but my parents raised me Christian and are rather devout. I am no longer a religious person, and I'm sure they suspect that as such. This is fine, since it's the truth. Here comes the touchy part... I think my mother would be very upset if she knew for certain and it would cause her a lot of undue stress for a long time. I wish to spare her this as much as possible, but they do keep probing me on why I don't attend church services. I don't wish to lie about it, but it does put me in a corner. Does anyone have any advice on either deflecting this question or a very careful way to let her know the truth? I don't want her agonizing over my immortal soul and whatnot, but I am rather confused on what to do here...