Atheism is not a belief system. The absence of something is NOT something. The absence of something IS nothing.
Our understanding of nature and ourselves is bounded by the limitations of our human language. Not many people study linguistics theory and just assume the words they are using mean a whole lot more than what they actually do.
There is a fascinating idea with regards to categories of dialectics called "Unity of opposites". The most basics of physical experience is the equation Time = mass - Energy. Without having opposites (mass and Energy) with some form of tension (the subtraction part of the equation) then you would not be able to experience time. All meaning in language comes from some form of a unity of opposites.
"This divine Logos, or law of the universe, centers around the idea of eternal flux, that things within the universe are constantly changing. Heraclitus explains this flux by examining the unity of opposites. It can be found that all things undergo transformations so that they may become their opposites. That which is hot will inevitably become cold. Every life is guaranteed death and with each death, there can be found new life."
Unity of opposites - Wikipedia
Here's a fantastic video on a lessons of non-duality:
Without duality you do not exist.
atheists have belief systems. everyone does. anyone identifying "their" system as atheist composes "their" set of beliefs; which are subjective on how to behave as sts or sto.
things may appear to be dualistic, or contrasted but in reality they are the same.
example E = m(c*c) are the same thing seeming to appear differently