I'm sorry that your parents have racist tendencies, but it's good that you are able to spot them. Many people that are raised that way pick up their parents' mentality without being aware of it. I am also adopted, but my parents are white and I am Asian. My parents are religious, but rather tolerant and liberal-minded. So my situation is nowhere near as extreme as yours.
I, too, stopped believing at some point and kept it a secret from my family. I am not even sure when I became atheist, it was sort of an evolutionary process that took place over several years. I would guess my full realization that I no longer believed happened early in college for me. Though, as a teenager, I never fully agreed with religion and was heavily agnostic. When I went to college, I found a lot of people like me. It was easier to deal with since I knew there were a lot of people around me that felt the same way.
My family has probably suspected, since I no longer attend church. And my mother finally confronted me about it yesterday. I decided not to lie. As expected, she was saddened and disappointed. I do not think she will ever seriously listen to my ideas on the matter. People get so stuck in their ways. She thinks something is wrong with me because of my non-belief, when that is certainly not the case. She also pointed to my atheism as being a reason that I have an inability to form relationships with other people, which is quite untrue in my case. She just doesn't know my friends. I think it will all die down and it will just be something we don't talk about, but not much will change.
Unfortunately for you, it sounds like that may not be the case. You will have to decide what's best for you. I completely understand not telling anyone because you don't want to upset anyone when there's no reason to tell anyone. I did it for years. But it can also be frustrating to hold your tongue and never feel free to speak your thoughts because it might make people uncomfortable. I especially don't envy you since it sounds like you have to deal with racist ideas from your own family. I am unsure of your living situation, but it sounds like you're with your parents. Once you are able to be on your own, it won't be as bad.
It might help you to find other people that don't believe, like you. It can be a relief sometimes to talk to others that think like you do and feel comfortable speaking your mind. To feel like you don't have to hold back for fear of upsetting other people, where you can be yourself. I was able to do this in college where I studied science and engineering, where disbelief is rather common. I also have many Christian/religious friends who are not bothered in the slightest with my atheism. And coming on to websites like this and being a part of an online community is one way to do this.
I am certain you will be able to find friends that are either agnostic/atheist themselves, or else are completely accepting of those who are. The world continues to progress, and more and more people are becoming accepting of those who are different, particularly in the younger generation. And non-belief and the non-religious will only increase as society becomes more educated, which gives me optimism. Just hang in there. Dealing with ignorance and judgement is something we all have to get used to. But there are others like you and you are certainly not alone.
Best of luck,