Catholicism, specifically. Mainly, she believes that everyone needs god, otherwise they'll lead empty lives.
My mother pitched a similar fit when I stopped being Christian. She originally insisted I attend church anyway. She later relented and told me I didn't have to go to church if I agreed to look into other religions (I was 12, so she had more power to mandate stuff like that.) It worked out okay.
I think she needed to know I wasn't lost in a moralless void, and eventually she could see that I still cared about ethics, other people, etc.
Yesterday my mom noticed (at a funeral) that I wasn't praying or participating in the mass. She confronted me about it, and I told her that no, I am not Catholic. Apparently it's sinking in and she's a bit hysterical... crying and screaming that she doesn't know me anymore.
What do I do? I don't know how to react/behave when she's doing this. It's like she wants me to feel like I'm doing something wrong, but it isn't working, so she's getting hysterical.
*edit*
I gave her a hug when she calmed down and told her I love her. That seemed to help, a little. Based on her previous behavior in situations where I've done something "wrong," she will (in the future) make barbs and slight passes at my beliefs. (Already she made one: insisting my love of science is causing my social anxiety disorder.) The advice given in the comments is really helpful. I plan on making the most of it.