Secular humanism, Marxism, nihilistic relativism and some forms of Buddhism are examples of belief systems that are atheistic. Odin worship, Hinduism and Christianity are a few examples of belief systems that are theistic.

So it’s rather silly for someone to ask for a defence of Atheism as a belief system in general, since it’s a single belief that’s a common denominator of many belief systems, just as it’s silly to ask for a defence of Theism in general, for the same reason.

But if an atheist criticises, say, Christianity, it’s not unreasonable for a Christian to ask “Well, what do you believe? Why’s your understanding of the world better?”