I am an atheist, and my mom and her boyfriend are VERY religious. I'm scared on how to tell my mum that I'm an atheist. What to do?!

Why do you feel a need to tell them?

No need to purposely start an argument keep it to yourself and if asked simply say I'm an atheist.

Be prepare for arguments, fights, mental anguish, yelling, threats, lies, screaming and name calling from you and your parents. 

Not sure how old you are .. But why do you need to tell them?  Unless you're a child and have to go to church with them .. It's really not their business.  But if you're secure in your beliefs .. Just be honest with them.  Maybe if they think you're serious about this .. It will be easier for them to accept.

There is a couple of schools of thought here.

First off and pretty importantly, Atheist have stayed quiet long enough. Now while I say this, it really applies more specifically to politics and the main stream public eye. Your situation is a more a "Core Family" issue. In this it doesn't really become an issue until it DOES. This means you comfort level with the onslaught of phrases such as "You should feel Blessed . . . ", "Why don't you pray about it . . . " or "Maybe that's how God made you." . . .

At some point you being uncomfortable with being "Lumped" under their belief umbrella may be you sacrificing "Your Voice" in this family . . .

That leads me to the other school of thought. If it means that much to you, communicating honestly shouldn't be a big deal if done for the right reasons. Doing it to hurt them is the wrong reason . . Doing it to establish "Your personal Voice" in this family is the right choice. You matter, your beliefs SHOULD matter . . . To you and to members within your core family.

Hope for the best, be prepared for the worst, but never count your beliefs as anything lesser then they would account their beliefs to.

Well, this is the equivalent of your telling a math professor that you no longer think that 2 + 2 = 4.

Do not expect them to be either indifferent to or approving of your choice.

Neither you nor they can "prove" that either is in touch with the actual reality that may or may not exist.

Personally, for me to become an atheist, it would be like giving up prime rib and Grand Cru Classe wine to live on bread and water---because I have found "the pearl of great price" mentioned in the Bible.

Deciding that God exists or does not exist leads to two entirely different philosophies of life---it is NOT like just preferring fried eggs to scrambled eggs---it is about whether eggs exist in the first place.

Good luck on your journey.

You are entitled to privacy.  There is no need to upset everything this way.  Say nothing about this in your family, go about your life making ethical and intelligent decisions, and make a contribution wherever you are, including at home.