A transcript of a segment of a podcast by Greg Koukl:
Atheism is the idea that there is no God.
It is not a 'lack of belief' in God. This is not what atheism is.
It does entail the idea that the atheist lacks a belief in God but they don't lack a belief about God. They are not without belief regarding the subject; they have a very firm conviction about it - a belief - and that belief is that God does not exist. When it comes to God there is none! That is a belief!
When I say 'belief' I don't mean 'mere' belief, maybe they have reasons for it, but a belief is just holding that something is so and that is one of those things.
People don't write books about non-beliefs and they don't hold lectures and have debates about non-beliefs.
They talk about their beliefs and this case atheists a belief that God does not exist which means they have no belief in God but they do have a belief about God.