Atheism is not an "Ism":
When people talk about "isms," they are referring to some "distinctive doctrine, theory, system, or practice" like liberalism, communism, conservatism, or pacifism. Atheism has the suffix "ism," so it belongs in this group, right? Wrong: the suffix "ism" also means a "state, condition, attribute, or quality" like pauperism, astigmatism, heroism, anachronism, or metabolism. Is astigmatism a theory? Is metabolism a doctrine? Is anachronism a practice? Not every word that ends in "ism" is a system of beliefs or an "ism" in the way people usually mean it. Failure to realize this can be behind other errors here.
Atheism is Not a Religion:
Many Christians seem to believe that atheism is a religion
, but no one with an accurate understanding of both concepts would make such a mistake. Atheism lacks every one of the characteristics of religion. At most, atheism doesn’t explicitly exclude most of them, but the same can be said for almost anything. Thus, it’s not possible to call atheism a religion. It can be part of a religion, but it can’t be a religion by itself. They are completely different categories: atheism is the absence of one particular belief while religion is a complex web of traditions and beliefs. Atheism is Not a Religion...
Atheism is Not an Ideology:
An ideology is any "body of doctrine, myth, belief, etc., that guides an individual, social movement, institution, class, or large group." There are two key elements necessary for an ideology: it must be a group of ideas or beliefs and this group must provide guidance. Neither is true of atheism. First, atheism is by itself just the absence of belief in gods; it's not even a single belief, much less a body of beliefs. Second, atheism by itself offers no guidance on moral, social, or political matters. Atheism, like theism, can be part of an ideology, but neither can be an ideology by themselves.
Atheism is Not a Philosophy:
A person's philosophy is their "system of principles for guidance in practical affairs." Like ideology, a philosophy comprises of two key elements: it must be a group of beliefs and it must provide guidance. Atheism is not a philosophy for the same reason that it is not an ideology: it's not even a single belief, much less a system of interconnected beliefs, and by itself atheism does not guide anyone anywhere. The same would be true if we defined atheism narrowly as denial of the existence of gods: that single belief is not a system of principles. As with ideology, atheism can be part of a philosophy.
Atheism is Not a Belief System:
A belief system is a "faith based on a series of beliefs but not formalized into a religion; also, a fixed coherent set of beliefs prevalent in a community or society." This is simpler than an ideology or philosophy because it's just a group of beliefs; they don't have to be interconnected and they don't have to provide guidance. This still doesn't describe atheism; even if we narrowed atheism to denying the existence of gods, that's still just one belief and a single belief is not a set of beliefs. Theism is also a single belief that is not a belief system. Both theism and atheism are part of belief systems, though.
Atheism is Not a Creed:
A creed is a "system, doctrine, or formula of religious belief, as of a denomination" or "any system or codification of belief or of opinion." Atheism is not a creed in the first sense for the same reasons it's not an ideology or philosophy, with the additional factor that that it has nothing inherently to do with religious belief. There are no atheist "denominations" and even narrowly defined it is not a religious formula. Atheism might appear as part of someone's creed in the second sense because a person might codify their positions, including atheism. Otherwise, though, atheism has nothing to do with creeds.
Atheism is Not a World View:
A world view is a "a comprehensive conception or image of the universe and of humanity's relation to it." This comes a little bit closer to atheism than anything thus far. Although atheism by itself does not offer any guidance on how to conceive of the universe and humanity's relation to it, it does exclude certain options — namely, those centered around some god. Excluding certain types of world views as options does not, however, qualify as a world view itself; at most, it might be part of a world view. Atheism is certainly not comprehensive in anything it might have to say, not even if defined narrowly.
Is Godless Liberalism a Religion?:
Liberalism” a religion should be recognized as an ideological attack rather than a neutral observation of facts. Sadly this is not the case and it has become far too common for critics of liberalism to claim that it’s inherently godless and religious, thus hoping to discredit liberal policies before they are even considered. The fact is, godless liberalism doesn't involve any of the basic characteristics common to religions: belief in supernatural beings, separate of sacred and profane objects or times, rituals, prayer, religious feelings or experiences, etc. Godless Liberalism Is Not a Religion...
Is There a Godless Church of Liberalism or Atheism?:
Ann Coulter and others have repeatedly used the label “godless” as a political smear. Because of their efforts, it’s become common in America to treat “godless” like a scarlet letter. Why would people who make a big deal out of being religious believers themselves consider it a criticism to accuse godless liberals of having a "church"? The truth is, there is nothing about godless liberalism that is church-like: there is no holy scripture, no churches or clergy, no cosmology, no higher power, and nothing else that is characteristic of churches. There is No Godless Church of Liberalism or Atheism...
Making Atheism More Complicated Than It Really Is:
The refutations of the above claims are all similar because the source of the errors is basically the same: people who describe atheism as as philosophy, ideology, or something analogous are trying to depict atheism as being much more complicated than it really is. All of these categories are defined in one way or another as systems of beliefs which provide guidance or information. None of this can describe atheism, whether defined broadly as the absence of belief in gods or narrowly as denying the existence of gods.
It's strange that this would happen because almost no one says such things about atheism's "opposite," theism. How many claim that mere theism, which is nothing more than a belief in the existence of at least one god, is all by itself a religion, ideology, philosophy, creed, or worldview? Theism is a common doctrine and it is commonly a part of religious dogmas. It's also commonly part of people's religions, philosophies, and world views. People demonstrate no trouble understanding that theism can be part of these things, but doesn't qualify as one all by itself.
So why do people fail to realize this when it comes to atheism? It's probably because of atheism's long-standing association with anti-clerical movements and dissent from religion. Christian theism has so dominated Western culture, politics, and society that there have been few sources of religious or theistic resistance to this domination. At least since the Enlightenment, then, atheism and atheistic groups have been a primary locus for freethought and dissent from Christian authority and Christian institutions.
What this means is that most people engaging in such resistance have ended up being pulled into the sphere of irreligious atheism rather than into an alternative religious system. Atheism doesn't have to be irreligious nor does it have to be anti-religious, but cultural trends in the West have caused atheism, irreligion, and opposition to religion to be drawn together in such a way that there is now a high correlation among them.
As a consequence, atheism tends to be associated with being anti-religion rather than simply the absence of theism. This leads people to contrast atheism with religion rather than with theism, as they should. If atheism is treated as the opposite of and opposition to religion, then it will be natural to assume that theism is itself a religion — or at least some sort of anti-religious ideology, philosophy, world view, etc.