I've made a list of the notable people in my church, to sort of expand on why I can't just wear a bright red atheist t-shirt one day and make a clean break of it. This is sort of my Magic 8 Ball response of what would happen when they find out, or why it would be bad, or something to that effect. Nicknames have been given to protect the innocent. (and me)
The Big Cheese: Probably blindsided. I'll admit, I've had to prevaricate on occasion about my “walk with the Lord” to keep myself out of suspicion. Funnily, even when I was still nominally a believer (in my head, I mean) I never really mentioned having any doubts, because I've gotten very used to playing the part of the church poster boy. I worry a lot about the disappointment thing, and the “you lied about your faith for how long?” thing mostly. I don't cope well with disappointing people. Also, it would bother me to start out on the “out” atheist path by having lied to pastor (but more having him find out about it) when one thing I'm going to have to deal with is the “amoral” misconception about being without religion.
Mrs. Cheese: We used to talk a lot more than we do now. We had a lot more in common in our lifestyles (coffee and books) than the Cheese and I, and we related better because of it. Now, though, I don't think she'd really expect to have noticed anything or seen it coming at all, but it would probably bother her that she was completely wrong to assume that I was still “fine in the faith” or whatever. And that I lied to her husband...
Jeeves: We never used to talk, and now we sort of do. I've played a lot on his assumptions that if I chuckled or nodded a lot it meant that we were in complete agreement, as opposed to me biting my lip really hard. Also I've had to make a couple cracks about hellfire and nonbelievers just to keep the conversation light, and those were, I think, especially convincing. But mostly I've turned into his (adopted) well-behaved, churchgoing son, and that's an unpleasant thing to lose.
Little Jeeve-etta: Same sort of family issue, except here I'm the well-behaved, churchgoing big brother. If I'm at all to be the good example I seem like now, then I know the Jeeves family will be worried that I somehow indoctrinated her with my evil fake-big-brother powers, and that'll be difficult to deal with if it comes along.
Mama Cheese: We talk quite a bit, and she's another one I've had to nod along to a lot. I think the biggest shock to her system will be that I don't vote with the party (read: the church) since that was a surprisingly strong assumption that I recently found out she had about me (and voting in general). Also, we have had a lot of time to chat informally, and the fact that my rather sudden and total disbelief never came up will probably be a bit of a nasty shock.
The Queen Mum: We don't say much more than hello once a week. Nonetheless, she's kind of the spiritual matriarch of the clan and if nothing else she'll be quite bothered that she was never dropped a note about the atheist in the camp. From time to time there'll be the “threshing” or sorting-themed prayer time, and everyone will be up in arms about there being sin or doubt in the camp, but apparently these were all resolved even though I sat there thinking “bullshit” through a sermon.
The Turtle: Very involved intellectually in this whole business. I think he's probably grown the most used to me acting like a good little choirboy and being willing to involve myself in things. Sadly for him, it's all part of the act, really, and I don't think he'll take the thinning of the ranks all that well.
Mrs. Turtle: Gushes a lot about God's plan for my life, specifically just how great it is that I'm following it. I tell you, you make one little slip about having an inkling about your “calling” when you're twelve and it haunts you for all eternity. Literally, as they think about it. Heh.
The Clan: They'll all be surprised at how well I had them fooled, if they notice at all. They'll be sad, disappointed, and other such unpleasant sensations, but nothing too specific, really.