I burn with envy as I read your post. :P
I hope to be as bold and happy with myself as you are one day. I probably will, one day soon.
I always think to myself "If they have an issue with me, that's their problem, not mine."
No, you're not the only one who hides your atheism from your grandma. I hide my atheism from your grandma too. ;)
Seriously, there are some things I do just to keep the peace. My parent in-laws have no idea that I am an atheist. I have no problem with it. Everyone is happy and I have no need to tell them. Now that might change if they start to force Catholicism on me. Then they will be in for a shock.
But for now, ergo bibamus!
I never thought I would be in the "closet" about anything. Even here in the heart of the Bible Belt, I normally would not hide who I am. However... my mother had a stroke two years ago and I returned home to care for her. (She is much improved in the meantime.) My mother and I share a general aversion to religion, evangelical denominations in particular after her upbringing in a fire, hell and brimstone Pentecostal church, as well as non-theistic beliefs. My sister, on the other hand, is a VeRy devout evangelical Christian who is convinced that homosexuals are taking over the world (wouldn't that be grand?!) and that Creationism is the ONLY logical way to explain existence. This was fine when I was living in San Francisco... a world away from these back waters. The problem I have now is that we are living in a home that my sister owns due to financial considerations. I do not feel that I can be open about what I (we) believe because we must keep the peace until we can find our own place to live and also because of the fragile health of my mother.
I think this is a very personal decision. I do not believe it right to think that people are misrepresenting themselves when they choose not to disclose some aspect of their lives... even an important one. My journey is my own. My life intersects with other people's - be they family, co-workers, friends, etc. - at different points. I am not obliged to share anything with anyone, even family. Those who have my trust, who love me - it is with them I share my deeper life. Someone like my sister is not capable of rational discussion and sharing of thoughts without becoming emotionally unstable. Just the fact that I will not go to church with her and that she feels she cannot talk to me about Jesus has created a barely tolerable tension. I choose not to put myself and my mother through that more than necessary.
My grandmother isn't a zealot. She's very open-minded for an older person. She'll be 92 this year.
I hide it from her, mostly because she's senile. She has early stages of Alzheimer's Disease. If I told her, she wouldn't understand, not because she's narrow-minded, but because she just doesn't have the capacity to. It would be pointless to bring up and it doesn't really matter anyway. I love my grandmother, so why cause any unnecessary confusion?