Myth:
When a belief has a unified group of adherents who change laws and exert force within society, it is recognizably a system because it is working as one. It is a belief system, de facto, and atheism is a belief system which atheists are trying to impose by force on society.
Response:
It is true that many atheists work together for different causes and seek some of the same sorts of changes or developments in society. Does this therefore mean that atheism is a "belief system"? Well, that would be like saying that "female" is a belief system because women work together for certain causes and seek some of the same changes in society. Is "female" a belief system? Of course not, that's nonsense — and so is the claim that atheism is a belief system.
The first and most basic error made in the above myth is to describe atheism as a belief at all. Some atheists do believe that some or any gods do not or cannot exist, but the broad definition of atheism itself is the absence of belief in gods. This myth basically tries to prove the truth of its conclusion by assuming the truth of almost the same thing: it tries to prove that atheism is a belief system by assuming that atheism is a belief. That's a logical fallacy known as Begging the Question.
We can ignore that, though, and still conclude that the myth is completely wrong. The fact that several people share something in common while working for some cause does not entail that whatever they share in common is a belief system, a belief, or is functioning as a belief system. This is obviously true of characteristics which are irrelevant to whatever cause people are working for, like simply being human or having hair, but it's also true of characteristics which are somehow connected to the cause, like being female, being a cancer survivor, or being a business owner.
One reason why this is the case is the fact that not all women, cancer survivors, or business owners join together for the same causes — not even causes that are logically connected to character trait in question. Not all women pursue feminist politics, not all cancer survivors worry about cancer research funding, and not all business owners oppose minimum wage increases. The same is true of atheists, of course. There are causes where many atheists are involved, but there are almost always atheists who support the opposite, and inevitably many atheists who just don't care enough to get involved at all. The reason is simple: there are no particular philosophical, political, social, or religious beliefs which follow automatically and necessarily from the absence of belief in gods.
Another error which the myth makes is to claim that atheists who are active in one cause or another are trying to impose atheism by force. When was the last time you saw an atheist seriously suggest putting "one nation under no gods" in the Pledge of Allegiance? This just doesn't happen because atheists are trying to impose atheism or any atheistic belief systems on society. At most, they simply seek to end government endorsement, support, or encouragement of religious beliefs — any religious beliefs, theistic or atheistic. Many religious theists agree with this and are themselves on the forefront of such efforts. It's no surprise that most church/state cases are brought by theists, not atheists: when the government privileges one religion in some manner, it inevitably puts other religions at a disadvantage.
Trying to convert others to a cause also doesn't make a shared characteristic a belief system. Women working together to promote feminist interests don't thereby transform being female into a belief system. Cancer survivors working together to promote greater funding of cancer research, and convincing others to support this, don't thereby transform being a cancer survivor into a belief system. All of this is trivially true and unquestionably obvious — pointing it out is a little like saying that water is wet and ice is cold. So why does it need to be said? Why do such obvious truths get ignored when it comes to atheism?
Some religious theists who repeat ideas like the above myth may mistakenly imagine that atheism is more important than other shared characteristics, but I suspect that many others are simply grasping at straws. They are desperate to portray atheism as a belief system or religion in its own right because this causes them to ignore actual atheistic belief systems, making it easier to attack atheists and atheism through Straw Men rather than address what atheists actually say, do, and believe. If such religious theists really wanted to address atheist beliefs, they'd spend time with atheistic philosophies like Humanism or Objectivism; this, however, wouldn't allow them to lump all atheists together and transform them into scapegoats for whatever they think is wrong with society today.
