A theist is someone who claims the existence of God can be proved. An agnostic is someone who claims the existence of God can neither be proved nor disproved. In order to differ from these two groups, an atheist must be someone who claims the ability to prove that there is no God.
As simple as this definition seems, those who claim to be atheists never claim the ability to prove that there is no God. Rather, their goal is to simply disprove the idea of the Christian God. So-called atheists have formed solely as the negative form of Christianity.
As a positive belief system, Christianity can exist, function, and flourish without atheism; yet as a negative belief system, atheism cannot exist without Christianity. Atheism has no thing to offer its adherents. What a terribly weak and pitiful thing to subscribe to.