DO. NOT. TELL. THEM.
Wait until you are out of the house. I've seen what happens to people who come out to tell their parents they're atheists. Sometimes they get treated worse than people who come out of the homosexuality closet, I am dead fucking serious. Wait til you are on your own first. And be prepared, too; don't be living somewhere else on your parents' dime and THEN tell them.
If you really want an idea of how they feel, ask them casually in a roundabout way how they feel about atheists first. Then make your decision based on their answer. Use common sense when doing so, too.
Wait until you are out of the house. I've seen what happens to people who come out to tell their parents they're atheists. Sometimes they get treated worse than people who come out of the homosexuality closet, I am dead fucking serious. Wait til you are on your own first. And be prepared, too; don't be living somewhere else on your parents' dime and THEN tell them.
If you really want an idea of how they feel, ask them casually in a roundabout way how they feel about atheists first. Then make your decision based on their answer. Use common sense when doing so, too.