You will hurt her much, much more by not telling her. You can probably trust her with the knowledge that you are an atheist.. Her being republican would have nothing to do with it.. If she is really in touch with God, then God can sustain her.. She might show herself to be very upset at first, and she just might pray for you. If there is no God who answers prayer, you needn't worry about it. If there is a God who answers prayer, you'll be grateful for her prayers when He brings you around to faith in Him! Actually, I mean this.. If you continue not to believe in God and she finds this out when you are 25 years old that you never trusted her enough to talk with her about it when you were 16, then she will be very hurt. I know. I'm a father of 5 adult kids, and though I haven't seen everything I've seen a lot. - Maybe way too much, in fact.