Unless your mother is a bible-thumping evangelical who is at church whenever the doors are open and condemns everyone who has different beliefs than her I would recommend just telling her. She probably won't be as upset as you might think. It would be easier than coming up with excuses not to go.
Only you know your parents. If you are dependent on them in anyway, then perhaps it is best if you keep quiet until you are out of their house.
It isn't just a case of 'my house, my rules' you have to worry about, it's the division of your family as a whole if they are fundies. If they really hold the belief that thinking differently will earn you an eternity in hell, a parent will stop at nothing to spare you that fate, even if it means making your life a living hell until you cave in and just go along with everything to spare yourself the trouble. Why get into it in the first place?
If you think they will respond in a rational, supportive way, then pick a time that has no stress, no outside variables that will give them the excuse that you are 'just going through a difficult time.' Stress the fact that you've felt this way for a long time now, and it doesn't mean you've changed in anything except this school of thought. Reinforce that you are still a good person, an good citizen.. whatever.
But if you have any inkling that things will go downhill....wait until you are out on your own.