Babies are atheists; they believe in no gods. Lacking belief is all that's required to be an atheist, not active disbelief.
http://en.wikipedia.org/wiki/Atheist Atheism is, in a broad sense, the rejection of belief in the existence of deities. In a narrower sense, atheism is specifically the position that there are no deities. Most inclusively, atheism is simply the absence of belief that any deities exist. Atheism is contrasted with theism, which in its most general form is the belief that at least one deity exists.