I am an aetheist and I don't believe that there is no god. Unlike religion to me it isn't necessary to justify whether there is or isn't a god, there just isn't one. Indeed atheists stray into beliefs if they go so far as to justify themselves but they don't need to so it isn't a belief system at heart.Aethiests try and deny that "lack of belief" is a belief system but that's like saying people who don't vote don't have a political opinion.
Simply put, If you criticise others for their belief then at that point aethism clearly BECOMES a belief system.
Each to their own.
For example, I do not believe that the sky is blue, it just is blue, I'm not willing nor do I need to argue and therefore justify myself with an opinion on why I think the sky is blue.
From my point of view those believing the sky is red would certainly have to justify themselves though! This is where religion fails because it absolutely requires more than ignorance on the part of beliefs and truth and this is dressed up as something called faith.