Tell them you're a mass murderer. Then, say you're joking and you're only an atheist. Their relief will be greater than their anger.

Someone already said this. Have people actually tried this in real life, that it has become such a popular method?

As well as atheism

I don't know, but it only makes sense

But seriously, just tell them. Perhaps you can let them know you are going on holidays somewhere and disappear for a couple of weeks, and the right before leaving you tell them and they have two weeks to think about it. You will avoid a violent situation. If you don't want to tell them straight you can write everything down and leave them the letter or something, that way you can explain that if they really love you they have to accept you and all that stuff.

You'd have to find some pretty extreme atheists to think that all people with religion are mentally ill. It's certainly not a majority of atheists who think that, most just think that religion is a belief not based on evidence.

Well, then probably they'll either forget it or you'll just go along with it. That's my bet anyway :P

In my opinion there's nothing you can do until you have moved out or until you feel the right time has come.

In my case, my mother knows that I'm atheist but my dad doesn't. Although it doesn't matter because I don't live with my dad.

When I get degree I'll definitely not care about my family's opinion on my beliefs.

Athiest have more open opinions, Relgious people think that god did this and no way around it. 

Well, most atheist tend to base their beliefs on something rather than on indoctrination.

Well, I start classes on Wednesday, and I have an exam on Thursday, so disappearing somewhere for a couple of weeks is out of the question. And to be honest, I am afraid they will say they dont love me anymore.