It is always hard when you are dependant on parents, and in those instances, I always suggest the same thing:
Stick with the lie, but try to avoid the conversations if you can. It isn't nice to lie to your parents, but if they are of the persuasion that your mom appears to be, you need to think about your security before you think about her. The last thing you want is her disowning you before you're even an adult.
Once you are at university, you can relax a little. You'll probably join a campus atheist group (I recommend it), so you'll have plenty of people to talk to about your problems. At this stage, since you aren't depending on your parents anymore, you can come out if you wish. If you do decide to come out, you need to push the issue of "it's my life", and "my beliefs aren't up to you", etc, whilst still maintaining that you respect the beliefs of your parents and would never attempt to ridicule them because of them.
Stick with the lie, but try to avoid the conversations if you can. It isn't nice to lie to your parents, but if they are of the persuasion that your mom appears to be, you need to think about your security before you think about her. The last thing you want is her disowning you before you're even an adult.
Once you are at university, you can relax a little. You'll probably join a campus atheist group (I recommend it), so you'll have plenty of people to talk to about your problems. At this stage, since you aren't depending on your parents anymore, you can come out if you wish. If you do decide to come out, you need to push the issue of "it's my life", and "my beliefs aren't up to you", etc, whilst still maintaining that you respect the beliefs of your parents and would never attempt to ridicule them because of them.