Just tell them. Their reaction is not your responsibility.

Ask yourself why you're afraid to tell them. Most people lie because they're ashamed of what they're doing.

We have to come out at some point because when we have kids we want to make sure that they don't indoctrinate our kids.

What is your motivation for "coming out"? To what purpose / end are you wanting to do it?

I never came out to my parents as I saw no particular point in disturbing their slumber. That was probably an easier decision for me because I lived about 2,200 miles away. Also it was easier because my parents, devout as they were, somehow understood the virtue of minding their own business.

I think my Dad would have been cranky about it and my mother worried ... but I am quite sure there would not have been a shit storm. Even so ... I saw no percentage in cluing them in. My Mom died in '98 and my Dad in '02 so it's a moot point now, but I have no regrets.

For one thing my private [un]beliefs are no ones business, not even my parent's. It's TMI. [shrug]

I have never "came out", but the stuff I post on facebook makes it obvious. One of my daughter's friends asked her if she knew I was an athiest. My mom to knows by the posts, but we don't discuss it. I guess it depends o the relationship you have with your parents. I would start off by talking about other religions they think are wrong. Ask them why they dismiss them without knowing what they are about. Then go with agnosticism. Or like everyone else suggested stay in the closet till they're to old to make you feel like your disappointing them, then tell them.

I would hope that their love for you both would be unconditional (as their teachings would have it) and that, like if either of you were gay, they'd accept you for every ounce of being that you two are; no matter what.
It's that unconditional love that they say they have for both of you, that will test them to the end. They will try to convince you that you're "going through a phase", or that "you're hanging with the wrong people". Neither, of course, is true, and you've both have obviously learned to question everything and to delve into logical reasoning.
It might be rocky discourse, at first, and it's going to take time for it all to sink in, but I think that in the end (if both sets of parents can live by the teachings that are shoved down their throats, on any given Sunday), you both will be okay.

I was lucky; my parents were both Christians, of different faiths, so my sisters and I gravitated to learn what we could about the "southern baptist" and catholic faiths....and it spurred us into learning about other religions and cultures. All three of us girls "moved on" from mainstream christianity, and delved into paganism, then agnosticism and then, finally, realizing that we're all connected on a molecular level and that there's no religion on this rock that could convince us that we were created by a malevolent omnipresence (what deity could possibly "love" but then throw a soul into hell for doing what this omnipresence already predicted we'd do...ya, okay. And don't get me started on the penguins and the mighty ark.) My parents more or less guided us into free thinking; they always told us that they didn't care WHAT we believed in, but only that we believed in something....therefore, we chose to believe in ourselves.

My advice? Don't do it. It's not worth causing family division over.