Atheism is a "lack of belief in god" and nothing more. No one can know there is no god for the same reason that no one can know there is no Santa Claus, Easter Bunny, unicorn, or other such creature. Some may say that we can know that there is none of the above since we can trace through history where these characters were created and that the ideas about them have changed over the centuries. The same can also be said of god (except the history of god goes back a bit farther so it is more difficult to track with certainty) but for this arguments sake, let´s say that such creatures, although there is a strong probability that they are fictional, can not be 100% positively proven unreal for the basic reason that the entire universe would need to be explored to positively assert the non-existence of such a hypothetical being.

If the theist attempts to define their god however, that specific god can be disproven through the use of logic and reason. In these cases, an atheist can accurately state that they know that the god described does not exist.