What do atheists believe?

Ilya Somin has a thought-provoking blog A Turing Test For Religion in which he says:

...simulating a Christian who is well-informed about the arguments for his religious views is a tougher challenge than simulating an atheist who is comparably knowledgeable about atheism.

The implication is that there is an atheist belief system corresponding a Christian belief system otherwise there would be nothing to be knowledgeable about.  Simply the absence of a belief in God is not the subject of much analysis.  So I added my comment and asked him: 

You say “… an atheist who is comparably knowledgeable about atheism.”

This is an interesting thought and one that I first passed over without parsing it since the atheists I have encountered (mostly on the Internet) have been very strident about their position. But I found their position to be expressed primarily as an opposition to Christianity. As a Christian I tended to define atheists as people who simply do not believe in a God.

Your formulation implies that atheism has a coherent philosophy regarding the non-belief in a non-material being who is the creator of … what is. If one is to be “knowledgeable about atheism,” it must mean that there is a belief system that we can be knowledgeable about. From this thread it appears that here is some dispute about what atheists believe. Does that mean that there are sect and varieties of atheists just as there are varieties of Jews, Christians, Muslims, et. al? Perhaps as a practicing atheist you can enlighten us non-atheists about this.

I'm curious about the answer. Are there meetings of atheists designed to reinforce their beliefs, and do those lead to schisms among the "congregants" who then go out to form their own sects?

UPDATE:  Ilya Somin replies:

There are in fact atheists who think that atheism is a systematic belief system that covers a wide range of issues. And some of them do hold meetings about it.

But I meant simply that, just as Christians put forward various arguments in favor of Christianity, atheists make various arguments in favor of atheism (i.e. - against the existence of God). A person who is knowledgeable about atheism would be familiar with these arguments, and be able to put them forward and defend them, much as actual athesits [sic]do.

Ilya, 

That’s fascinating. I’m curious what forms the basis of an atheist belief system other than the absence of God. From the perspective of a Christian, I can understand what the absence of an eternal creator and judge and implies, but I’m curious what this implies from the perspective of someone who rejects these positions. Is there something, other than humanity or science that takes the place of the God that is at the center of a Christian ethical system? Put more simply, to an atheist, what is the difference between Mother Theresa and, say, Joe Stalin? They are both dead and from the perspective of the universe, both of them were insignificant bits of protoplasm that lasted for a few years and are in the process of being recycled. And if there is no difference, why not emulate Stalin. He had a better life than Theresa.   [edited]