Do you really think of your parents that lowly? In my experience, which isn't much to go on, admittedly, most seem to really underestimate their parents. You shouldn't necessarily assume that your parents are simple-minded just because they've got some beliefs indoctrinated into them that they don't question (even you do - your brain couldn't function without some assumptions to go on, I suspect), though, obviously, you know them far better. I really suggest you tell them plainly and respectfully that you don't believe in Christianity and haven't for a while and see how they take it.
I've never been in a similar situation, though. While I'm not even sure enough of what my beliefs are to question them, my parents are very open-minded about everything and would be very eager to discuss such subjects if I wanted to become an atheist.
“These are my principles; if you don’t like them, I have others.” – Groucho Marx
