As far as I can tell, atheism is not a "belief system".
It would seem that an atheist is just a person, by personal choice, who don't believe in the existence of any deity...be they be "one" or "many"...
The term "system" would suggest that it has many different parts (or many different teachings) that are linked or connected in some ways together as a whole.
I am not sure that I am making sense in what I am saying above.
So to rethink what I am saying, the average atheists would simply state that "I don't in God". These wouldn't need to say or believe in anything else, other that it is his choice to "not to believe".
These atheists don't require to follow any champion of atheism. He doesn't require a "system" of teaching to make his own decision. These atheists don't require to follow any teaching of atheism from atheist teachers, or he doesn't need to follow any philosophy of atheism from philosophers.
Truth be known, I don't think much about philosophers, because a philosopher have the tendency to overthink what atheism is.
Either a person believe or don't believe, he doesn't need philosophy to be an atheist. He can, but it is really not necessary.
To give you an example. One of my cousins is atheist. He doesn't read any book about atheism, written by an atheist. He doesn't watch YouTube video about people discussing what atheism is. He doesn't attend lecture or convention for atheists. He doesn't care what other atheists have to say about atheism.
A "belief system" sounds like following certain protocols, learning about teachings about atheism from other atheists. And even having a "leader" among atheists.
I hoped that make sense...but when I think about what I have written, I don't think I make sense at all.
Oh, dear.