Given the ambiguity of language, there are many "opposites" for the first sentence. A strict logical negation would be "it is not the case that atheists believe that there is God". In its weakest form, this could mean that they have never been exposed to the concept of God so they cannot have a belief of something of which they have no knowledge. The more conventional "opposites" are "Atheists do not beleive that there is God" and "Atheists believe that there is no God". These are sometimes called weak and strong atheism. I personally fit into the weak atheist category. I do not believe that there is any god because there is no evidence. Some call this agnostisticism.