Atheism ultimately is the argument that "I haven't heard a sound argument for a God from anyone so I maintain the default position of nothing on the subject."
Actually, that's the agnostic position - "There's not enough data for a rational determination of the existence of God, therefore the I hold no opinion as to whether God exists or not". Atheism is a positive assertion of the non-existence of God.
Well, that's the rational definition. The kooks at American Atheists are always pissing themselves about how agnostics are just weak-willed atheists or atheists without a backbone. They play the same kind of trick that Southern Baptists do. American Atheists define anyone who does not have a positively asserted belief in God as an atheist. So, in their view, agnostics, most buddhists, daoists, and even babies are all atheists. It's just a game to make their numbers look a lot larger than they really are.
The reason I say this is like the Southern Baptist strategy is that Southern Baptists will claim in one argument that there are over 1.3 billion Christians in the world. Then, in another argument, they will say that Catholics (about 1 billion right there), Eastern Orthodox and other Eastern churches like the Nestorian and Coptic churches (assuming they've even heard of them since a lot of them don't believe in this place call "outside the USA", unless it's somewhere we bomb, but that's a part of their double-think), Lutherans, Methodists, Episcopalians, Anglicans, and just about everyone else that is not them are not really Christians. They'll claim 1.3 billion Christians in the world and then turn around and say that about 1 billion Christians (Catholics) are agents or dupes of Satan.