I decided after a couple years of consideration that I was an athiest. Then, after I decided I couldnt lie to my parents anymore, I told them. My parents are divorced and my moms side of the family is very religeous. My dad is an atheist. I tried to tell my mom that I wasn't cristian and she got very quiet. Now every time the subject of religion is brought up she either quiets down again or gets upset and starts asking about how I could possibly not believe in a god. My brother is also upset with me about it but he leaves kit alone most of the time. When I told my dad he was overjoyed but now he makes very rude and derogatory jokes about theists that I'm not comfortable listening to. I think my mother and brother believe I will do the same thing as my father. I told my father I didn't want him to tell any more of the jokes but he just got very angry with me. Now both sides of my family are upset with me and I don't how to fix it. Can someone tell me what to do?

I feel you, I'm an atheist kid of two strictly Christian parents.

I think the only thing you can to do to help the case with your mom is to try to make her understand that your lack of faith (correct me if I'm wrong, but this is the case with most atheists?) comes from a lack of proof. And yes, you can't prove that God /doesn't/ exist either, but that isn't a claim that atheism makes. Your mom doesn't have to fully understand what you tell her, but she does have to respect what you tell her. Just as I'm sure you respect her religion, she should respect that you lack one.

Try to make it clear that you don't have anything against Christianity, and that this isn't you betraying her or the religion. This isn't you trying to hurt or dishonor anyone. It's just that you think differently from her, and that might be something she has to get used to but she should at least try to accept the idea. Acceptance is something all mothers should be able to give their children.
You can also mention that you aren't looking to follow in your dad's footsteps (though I'm not sure what he did that was so bad?), and that you still want to keep a healthy relationship with your mom and the rest of your family. Be open with her, and urge her to be open with you.

If you haven't been confirmed yet, maybe you can offer to at least follow Christianity until your confirmation. Afterwards, the idea is that you're a full christian so you should be able to choose how you practice (or not to practice at all). If not, then when you're no longer living under their roof (or if you already are out of their house, then there's all the more reason for them to let you think for yourself).
This is the closest thing to a compromise that I was able to make with my parents. Also, if they're worried about keeping face, just don't say that you're an atheist unless you're asked by someone directly (and even at that point, if you don't want to kick up any dirt, you can still lie).

As for your father, once he cools down you can try to explain to him again that religion is still a sensitive subject for you. Maybe you can joke about it in the future with him, who knows. But right now, nope. He'll probably be at least a little salty about it, but he should get used to this too.
As an atheist with a group of atheist friends, we make jokes about Christianity. A lot. I can tell you honestly we don't mean any harm, it's just our way of coping with the fact that we're rejected by a lot of Christians. I'm not saying we'd tell you these jokes knowing they made you uncomfortable, but I just wanted to let you see a different perspective of it.

Anyway, I wish the best to you. I hope I was of at least a little help here. xx

Are you sure that's a big problem? I understand your mother's perspective, but from what I can collect, as long as you avoid the topic, it should be fine. It's a sensitive issue that does not have to be talked about all that much.
As for your dad, just telling him that voicing his opinnion while he's mocking others' is really contradicting. He should be sensible enough to understand that, he was married to a christian after all.

Also, are you sure you're an atheist and not just agnostic?

I am a Cristian but my mom disapproves of my friends who are mostly athiest. One of them has Cristiano parents and she's athiest so if you just stay the way you are then it will become a normal thing for everyone.