So when someone asks you what do you belive in, what would you say. They are asking you what you believe in, not what you don't believe.
I dont think it was such difficult to understand what I said.
When someone asks you what do you believe in (as faith);
If you are a theist, you will say "I believe in God" or you can say the religion which you follow.
If you are an atheist, the answer varies from person to person, you cant say "I dont believe in anything, I'm an atheist", because its the same thing with sayin "I dont believe in God, I believe there is no God"..............illogical.
If you are an agnostic, you can say "I'm a bit confused" : )