Atheists don't have any particular belief system or an ideology of sorts. Recently discussing this with a friend I faced queries like why then was this suffix 'ism' in atheism, or what atheists believed in if there was no belief system in atheism? “That means you don’t believe in anything” was his reaction and then said in a lighter vein, “Nihilist perhaps”.
Atheism to me doesn't mean a strict and codified doctrine of some faith or a divine revelation. It should also not mean to imply an anti-god decree or ‘ism’ - a reactionary phrase in other words. Atheism means non-theism or absence of theism. Theism in itself is not any particular religion but very much part of every organized religion. Theism is a belief in god / gods while atheism means absence of that. The same way atheism can be a part of an ideology or philosophy. For example, Indian philosophy has atheism as an integral part of it.
Similarly, it is not a belief system following any tenets laid down by some god or his prophet. It is not like believing that god finished off creation in six tiresome days and took a well deserved rest on the seventh. Or like this one, contrary to all sensibility, that a virgin gave birth to some god's son. Or that all those non-believers would be put in the fire of hell once their prophet returned to this earth a second time.
Atheism is not an ideology (no holy book, no commandments, no prophet or god, no prayer, no sword, no Kalashnikov). My atheism is a knowledge system based on science and scientific methods of inquiry, experimentation and understanding. I don’t just believe it, but in fact I am a material product of this nature evolved during the continuous process of evolution. I don’t have a, what people call, soul or any other feature within me which isn’t physical or physically caused.
My morality and behavior emerge from a value system which I have developed genetically/biologically and/or imbibed from the environment and natural surroundings that as a human being or social animal I have to adapt to. Similarly, I have no free will to cause any action of my own and all my actions are a result of interaction with the environment around me. This absence of free will doesn’t mean presence of something predestined.
Atheism is a synergy between me and nature based on my behavior and actions in consonance with nature’s unpredictable laws and ever changing environs. The more I synchronize with nature the better I understand atheism.
In short atheism is nature; nothing more nothing less