Atheism seems to be on the table these days here at UD and a few points need clarification.
First up, what is Atheism?
The usual dictionaries are consistent:
atheism
n. Disbelief in or denial of the existence of God or gods.
[French athéisme, from athée, atheist, from Greek atheos, godless : a-, without; see a-1 + theos, god; see dh?s- in Indo-European roots.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.atheism
n (Philosophy) rejection of belief in God or gods
[C16: from French athéisme, from Greek atheos godless, from a-1 + theos god]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014a•the•ism
n. the doctrine or belief that there is no God.
[1580–90]
Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.atheism
the absolute denial of the existence of God or any other gods.
-Ologies & -Isms. Copyright 2008 The Gale Group, Inc. All rights reserved.
However, from at least the 1880’s, there has been a claim by some advocates of the same, that what is meant is someone without faith in God.
(This tends to serve the rhetorical purpose of claiming that nothing is asserted and it can be taken as default, demanding that theists provide “compelling” warrant for faith in God. Where, often, this then leads to selectively hyperskeptical dismissals, sometimes to the degree of claiming that “there is no evidence” that supports the existence of God. [Of course, the no evidence gambit should usually be taken as implying ” there is no evidence [that I am willing to acknowledge].” Through that loophole, as fair comment, a lot of clearly question-beggingly closed minded hyperskepticism can be driven.)
There are many varieties of atheists, including idealistic ones that reject the reality of matter. However at this juncture in our civilisation, the relevant form is evolutionary materialistic, often associated with the scientism that holds that big-S Science effectively monopolises credible knowledge. (Never mind that such a view is an epistemological [thus philosophical and self-refuting] view. Evolutionary materialism is also self-refuting by way of undermining the credibility of mind.)
A key take-home point is that atheism is not an isolated view or belief, it is part of a wider worldview, where every worldview needs to be responsible before the bar of comparative difficulties: factual adequacy, coherence, balanced explanatory power. Likewise, given the tendency of modern atheism to dress up in a lab coat, we must also reckon with fellow travellers who do not explicitly avow atheism but clearly enable it.
So, already, we can see that atheism is best understood as disbelief — NB, Dicts: “refusal or reluctance to believe”/ “the inability or refusal to believe or to accept something as true” — in the existence of God, claimed or implied to be a well warranted view; not merely having doubts about God’s existence or thinking one does not know enough to hold a strong opinion. It inevitably exists as a part of a broader philosophical scheme, a worldview, and will imply therefore a cultural agenda.
(I add: Note by contrast, AmHD on agnosticism: “The belief that the existence or nonexistence of a deity or deities cannot be known with certainty. “ Where, of course, certainty comes in various degrees, starting with moral certainty, and where knowledge, as commonly used often speaks to credibly warranted beliefs taken as true but not typically held as utterly certain beyond any possibility of error or incompleteness. We not only know that 2 + 3 = 5, but we claim knowledge of less than utterly certain facts and theories. For instance, in the mid 2000’s, the previous understanding and “fact” that Pluto was the 9th Planet of our solar system was revised through redefining Pluto as a dwarf planet.)
It will be further helpful (given objections that suggest inapt, distorted caricature) to excerpt from the Stanford Encyclopedia of Philosophy, as appears at comment 11:
“Atheism” is typically defined in terms of “theism”. Theism, in turn, is best understood as a proposition—something that is either true or false. It is often defined as “the belief that God exists”, but here “belief” means “something believed”. It refers to the propositional content of belief, not to the attitude or psychological state of believing. This is why it makes sense to say that theism is true or false and to argue for or against theism. If, however, “atheism” is defined in terms of theism and theism is the proposition that God exists and not the psychological condition of believing that there is a God, then it follows that atheism is not the absence of the psychological condition of believing that God exists (more on this below). The “a-” in “atheism” must be understood as negation instead of absence, as “not” instead of “without”. Therefore, in philosophy at least, atheism should be construed as the proposition that God does not exist (or, more broadly, the proposition that there are no gods).
This definition has the added virtue of making atheism a direct answer to one of the most important metaphysical questions in philosophy of religion, namely, “Is there a God?” There are only two possible direct answers to this question: “yes”, which is theism, and “no”, which is atheism. Answers like “I don’t know”, “no one knows”, “I don’t care”, “an affirmative answer has never been established”, or “the question is meaningless” are not direct answers to this question.
While identifying atheism with the metaphysical claim that there is no God (or that there are no gods) is particularly useful for doing philosophy, it is important to recognize that the term “atheism” is polysemous—i.e., it has more than one related meaning—even within philosophy. For example, many writers at least implicitly identify atheism with a positive metaphysical theory like naturalism or even materialism. Given this sense of the word, the meaning of “atheism” is not straightforwardly derived from the meaning of “theism”. . . . .
[A] few philosophers and quite a few non-philosophers claim that “atheism” shouldn’t be defined as a proposition at all, even if theism is a proposition. Instead, “atheism” should be defined as a psychological state: the state of not believing in the existence of God (or gods). This view was famously proposed by the philosopher Antony Flew and arguably played a role in his (1972) defense of an alleged presumption of “atheism”. The editors of the Oxford Handbook of Atheism (Bullivant & Ruse 2013) also favor this definition and one of them, Stephen Bullivant (2013), defends it on grounds of scholarly utility. His argument is that this definition can best serve as an umbrella term for a wide variety of positions that have been identified with atheism. Scholars can then use adjectives like “strong” and “weak” to develop a taxonomy that differentiates various specific atheisms. Unfortunately, this argument overlooks the fact that, if atheism is defined as a psychological state, then no proposition can count as a form of atheism because a proposition is not a psychological state. This undermines his argument in defense of Flew’s definition; for it implies that what he calls “strong atheism”—the proposition (or belief in the sense of “something believed”) that there is no God—is not really a variety of atheism at all. In short, his proposed “umbrella” term leaves strong atheism out in the rain. [–> which makes little sense]
Although Flew’s definition of “atheism” [thus] fails as an umbrella term, it is certainly a legitimate definition in the sense that it reports how a significant number of people use the term. Again, there is more than one “correct” definition of “atheism”. The issue for philosophy is which definition is the most useful for scholarly or, more narrowly, philosophical purposes.
We can go further.
For, we all have intellectual duties of care in general and as regards worldviews and linked cultural agendas. There are particular, inescapable associated duties to truth, right reason, prudence (including warrant), sound conscience, fairness, justice, etc. To see why such are inescapable, consider the consequences of a widespread rejection of such duties: ruinous chaos that would undermine rationality itself. Reason is morally governed.
Also, given that post Godel, not even sufficiently complex mathematical systems are subject to proof beyond doubt, that one cannot provide absolute demonstration is not at all the same as that one does not have adequate warrant to hold responsible certainty about key points of knowledge. In this context, the issue is reasonable, responsible faith in a credible worldview. Where, the claim one has “absence of belief in” God is often patently evasive. Why such a strange lack?
Could it be that one knows enough to realise that trying to disprove the reality of God is an almost impossible task, once there is no demonstrable incoherence in the theistic concept of God? (Where, we note, that the old attempt to use the problem of evil to lead to such a contradiction has failed; a failure that is particularly evident, post-Plantinga.)
Now, such is significant, especially given point 7 from the recently cited six-country study on atheists:
7. Also perhaps challenging common suppositions: with
only a few exceptions, atheists and agnostics endorse
the realities of objective moral values, human dignity and
attendant rights, and the ‘deep value’ of nature, at similar
rates to the general populations in their countries. (3.1)
A key to this, is the already mentioned point that our mental lives are inescapably under moral government, through undeniably known duties to “truth, right reason, prudence (including warrant), sound conscience, fairness, justice, etc.” The attempt to deny such rapidly undercuts rational discussion and the credibility of thought and communication, much as is implicit in what would happen were lying to be the norm. So, one who rejects the objectivity of such duties discredits himself.
However, it is also possible to hold an inconsistency; accepting objective morality but placing it in a framework that undermines it.
A start-point is to see that our rationality is morally governed through said duties. This means, our life of reason operates on both sides of the IS-OUGHT gap, requiring that it be bridged. That can only be done in the root of reality, on pain of ungrounded ought. And no, indoctrination, socialisation and even conscience do not ground ought. We need that the root of reality is inherently and essentially good and wise, a serious bill to fill.
You may dispute this (so, as a phil exercise, provide an alternative _____ and justify it _____ ), but it is easy to show that after many centuries of debates there is just one serious candidate: the inherently good, utterly wise creator God, a necessary and maximally great being. One, worthy of loyalty and of the reasonable, responsible service of doing the good that accords with our evident nature. This is the heart of ethical theism.
There is another angle. How much of reality do we know, how much of what is knowable do we actually hold, and how much of that is certain beyond future correction? The ratio is obviously trending infinitesimal; even dismissing Boltzmann brain scenarios, Matrix worlds and Plato’s cave worlds etc.
So, what if what is required to know God is, is beyond what one happens to know, or what one is willing to acknowledge?
In short, the positive affirmation that there is no God is arguably an act of intellectual irresponsibility, given our inability to show that being God is incoherent and our effectively infinitesimal grasp of what is knowable.
Let me add a table, as a reminder on logic of being:

Indeed, as it is easy to see that reality has a necessary being root (something of independent existence that therefore has neither beginning nor end), given that traversal of the transfinite in finite temporal-causal steps is a supertask and given that were there ever utter non-being, as such has no causal powers that would forever obtain, if a world now is, something thus always was. Thus, too, the question is: what that necessary being is, and that is further shaped by our being under moral government starting with our rationality.
Where also, a serious candidate to be a necessary being either is, or is impossible of being as a square circle is impossible of being. Where, a necessary being is a world-framework entity: a component of what is necessary for there to be any world. God as historically understood through theism is clearly such a serious candidate (if you doubt, kindly justify: ____ ), and so the one who poses as knowing that God is not implies having warrant to hold God impossible of being. Where, given the centrality of root of reality, ducking the question is clearly irresponsible.
In short, asserting or implying atheism requires a serious — and unmet — burden of warrant. END