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. I am an athiest. Apparently it's sinking in and she's a bit hysterical... crying and screaming because I don't pray- insisting that she doesn't know me anymore.

You can't help her unless you've correctly diagnosed the reason why she's crying, screaming, and being hysterical. You mentioned a funeral. Are you sure this is really about religion?

As it does for grouchymusicologist, the analogy to coming out queer jumps out at me. (When I came out to my mom, she replied "Well, I don't like that at all!" I replied "Yes, I didn't expect that you would." The conversation ended shortly thereafter.)

I pretty much endorse everything everyone here has said, but emphasize the "Don't Panic" response. Stay calm, stay polite, stay centered. A few more thoughts:

• It is not your job to control your mother's reactions, or her feelings. You are not obligated to feel or think the way that would make her happiest, but neither is she obligated to feel or think the way that would make you happiest. Right now you both feel and think in ways that make one another unhappy, which is unfortunate, but it's something that happens in relationships.

• Remember that this, too, shall pass.

• Stay aware of opportunities to improve your relationship with her that don't involve giving up your own integrity, take them when you see them, ignore opportunities to make your relationship worse whenever you can.

• Remember that you have a lot of power here, although it's highly constrained power. Be consistent in how you use that power and over time you will affect your environment. Reward the behavior you want; ignore the behavior you don't want; over time the pattern of behavior will change. But consistency is key. If you let her control the interaction -- for example, if you end up having an argument when an argument doesn't serve your purposes, simply because she did something that upset you -- then you give up consistency, and you give up those benefits. (If she is consistent and you are not, over time your pattern of behavior will change.)

A good heuristic (whatever else is going on) is to never argue when emotions are high, instead focusing on restoring the calm. Only talk seriously (i.e. present your abstract arguments or build up the requisite concepts for communicating future arguments) when emotions are in check.

If it's a hopeless project to change someone's mind (as it seems here from what you wrote in the comments) and you're not in a position to enforce your own authority, work on developing tolerance and avoid brandishing the behaviors (statements) that trigger intolerance (while tolerance is not yet developed, which may be never). For example, when asked to justify "objectionable" beliefs like atheism, you may try invoking relativistic arguments or uncertainty to belittle your own position (moving it to a closed magisterium, revoking its pretensions of universality or objectivity) and to "accept" their position (or "point of view", in more relativistic language), instead of defending your own position. In other words, present your "objectionable" beliefs as low-status, so that a higher-or-similar-status person doesn't feel threatened (offended) by them and doesn't hurt you (in some sense) as a result.

insisting that she doesn't know me anymore

To take the edge off it, among other things, you may want to assure her that your values have not changed, so she does know you almost as well as before. You still love her and the rest of the family, you still grieve for the person who passed away, you do not steal or lie or "sin" in whatever other ways, at least not any more than before your deconversion. Stay friendly and positive, ignore any hurtful remarks you will get (if you have trouble with it, think about it from her point of view: her world has just been turned upside down, no wonder she has trouble staying calm).

Once she is ready to talk, you can find out whether your faith is more important to her than, say, your integrity (discussing an example of a devout Catholic who committed a transgression can be useful there). Encourage her to discuss the issue with her pastor, maybe. Under no circumstances you should be sucked into discussing the merits of Christianity, not at this stage.

It happens on occasion that a person considers atheism (or homosexuality) a sin worse than murder, in which case you may end up severing your relationship for a time.

It would be useful to others in similar positions if you came back to this after some time and said what you did and how it went. Maybe a month or a year.

If she doesn't understand how you could come to reject Catholicism, perhaps it is true that she doesn't know you and maybe never really did. You could explore her model of you with her and find out who she thinks you were and who she thinks you are now. What is it, specifically, that she is scared of? Does she have some rational fears or are they all irrational? For example, is she afraid that you're going to go to Hell when you die (irrational fear) or is she afraid that your philosophical differences will strain the relationship (maybe more rational)?

I would recommend the following steps:

1) Find her objections 2) Assure her that you understand that her objections feel valid

You can, if you like, try to explain why her objections don't feel valid to you (unless of course, they do) but try to keep the conversation away from facts, premises, or rationality. The odds are very high that she is not becoming hysterical about you becoming an atheist because of some rational process. The odds are also very high that she cares more about feeling listened to than about your atheism. Try to steer clear from why you are an atheist: don't invite an argument where there doesn't have to be one. It likely won't be a productive argument if you have it. Definitely steer clear of directly attacking Catholicism: the odds are high that she has heard such attacks before and it could cause a negative social grouping that it doesn't sound like you want.

I'll leave you with one thing that can help if you become frustrated with her that I learned from sales training: objections=interest. It can feel like your mom doesn't value your welfare or well-being, especially if you believe religion was doing bad things for you. But if she genuinely didn't value your well-being, she would be unlikely to bring up an objection to your atheism. She might be mistaken about what is good for you, but she's likely very scared and doesn't know how to handle this. Anything she says that is hurtful is probably best attributed to that fear.

What do I do? I don't know how to react/behave when she's doing this.

Walk away. Definitely don't engage or react emotionally, that just rewards the codependency.

The other option it may be too late for: Lie. When in Rome do as Roman (Catholics) do. Sharing details about your models of reality is something you do for your own benefit with people who can interact with it in a way you consider useful. It's not something you are obliged to do with people who are unstable, controlling and ineffectually manipulative.