My earlier atheist's assertion that "I know there is no God" emerged as the least defensible. As the British writer G.K. Chesterton famously remarked, "Atheism is the most daring of all dogmas, for it is the assertion of a universal negative."

Atheists subscribe to a certain world view that includes numerous beliefs about the world and what is in it.  Theists say that there is something else that also exists - God.  If theists did not exist, atheists still would, but perhaps there would be no special name for them.  But since theism has become so dominant in our world, with so many people believing in God or gods, atheism has come to be defined in contrast to theism.