From your post, I don't know how old you are or what church your family attends. If they are middle of the road Methodist, presbyterian, or one of the "mainline" churches, you may get away with a certain amount of rebellion without being sent to some jesus camp to have belief pounded into you. If they are fundamentalists, they may become abusive to force belief into you. Keep your head down. Try to be polite.
No one can force you to be a christain. Even according to their own tenants of faith, belief must be voluntary. Be true to yourself in your heart even if you can't be vocal about it. I know it seems like living a lie, but right now the law is on their side and your safety and well being is the most important thing.
I'm feeling somewhat disturbed about so many people giving advice to keep her head down lest she be abused or sent to a jesus camp. No one can force you to believe what you don't believe. If you are under any threat of violence to obey, OP, then you have every right to call Child Protective Services. Parents may be the bosses for now, but they never have the right to be violent for any reason.
It would be good if you could give us some more information, Sono Gosu Shojo, what you told your dad before he said that to you, and what type of services your family attends. Your profile says you live with your grandma, so I'm not sure how often you even see your dad.
It's good that he did not take your Internet privileges away from you, so you can keep coming here for support. There are online books you can read. Depending on how old you are you can go to your local library with a friend and read atheist books there. Or if you have a friend at school who is an atheist, or supportive, you can go to a bookstore together to look at books.
I wish there were more resources for kids and teens who don't share their parents' faith. It's really unfair to force a religion down young people's throats, because we are so impressionable then - I know I was. It's brave of you to have even said anything to your parents.
I was a bit rebellious myself as a teenager, and I would fight with my dad about my religious choices. Sometimes I got out of going to church and other times I felt forced, but I'd sit there with my headphones until they realized what I was doing. But then again, I was a fighter, I knew I'd get hollered at, and I didn't care. Now I've mellowed, and I even like to avoid arguments. At this age, I have a better relationship with my dad. He doesn't think it's cool to be an atheist, but he doesn't try to convert me or anything.
I wonder if there's some type of a support group that's specific to minors.
Remember that no one can ever tell you what to think. Your mind is free, and what you have in front of you are decisions. You probably know what your dad's reaction will be depending on what actions you take, and you have to think, can I live with this reaction? Will it make my life better/worse/stay the same? What is better living a lie or living with the consequences of living as myself? Whichever seems like a better path, is the one you will probably end up taking.
I'm with New Atheist. If you just shut up and do as you're told, well that leads to personal issues. Say "fuck it" and put it all on the table. One thing I've learned in the past decade is that living safely is a great way to never live at all, and by not really living we spend the rest of our lives wishing we had. I can at least say that I stood up to my family. Shunned by the church, but accepted by the real friends. I never had the fake ones. Be real and real friends will show up. Be real and a real life is set before you. Be real, because that's the only you there is. And if they get abusive, well, not gonna say how I'd handle that (I'm a better fighter than my dad and all brothers, but I do hate violence), but there are so many ways to deal with it. If you have the nuts to stand up, have the nuts to go all the way. Just be ready. I know this shit is hard, a lot of things in life are. But I've learned to deal with what's hard now (I know this is sounding wrong!) and take on the next issue as it comes (oh hell) along. But if I set things aside, then I have fifty issues to face.
I don't know, just trying to sound intelligent at 1am. Hope I've made some sense. :)