The term “non-theist” already exists to denote individuals who merely lack belief in God.1 This is relevant for three reasons.
In academic settings (peer review etc.), atheism is consistently understood as the belief that God does not exist.
After all, consider…
This is relevant both for lay conversations using the term and for academic venues.
Relevant for lay discussion: In most conversations about God's existence, the level of precision would benefit from reflecting academic discourse because the terminology as established has been established this way for a reason. (See above.)
Relevant for academics: In academic conversations about God, shifting the meaning would introduce new frustrations and confusion for academics. Students and professionals who are reading and quoting peer-reviewed material would need to be very sensitive to the sudden grand shift in meaning, and therefore to the dating of the source they are quoting. (Is this usage instance pre-shift or post-shift?) Likely necessary would be swaths of footnotes to elucide for readers the meaning of atheism as used in this or that quote.
Theism (as classically defined) and atheism are contradictories, such that if theism is true, then atheism is false (and vice versa).
This is relevant because on the revisionist definition, “atheism” is a non-view statement about a cognizers psychological state. So ultimately “atheism” cannot be false and doesn't even attempt to answer the question of whether God exists.1 As explained by a prominent atheist (Paul Draper) in,
The Standford Encyclopedia of Philosophy: “‘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).”
The revisionist “lack of belief” definition of atheism is overtly ambiguous.
After all, there would suddenly be two ways to be an “atheist,” by either…
(a) …affirming the proposition <God does not exist>, or…
(b) …withholding belief about it (traditionally called “agnosticism”).1
This is relevant because the imprecise usage would in turn needlessly require interlocutors to spend extra time discerning which of those two remaining options apply, rather than the individual being straightforward about his position from the get-go. It also can lead to equivocating over so-called strong and weak atheism in discussions, which can be used in misleading ways2 Right now, things are efficient: One simply says whether they are a theist, atheist, or agnostic. Simple.3
The Greek roots of “atheism” yield “not theism.”1
• Cambridge Dictionary of Philosophy 2nd ed.: “_(from Greek a-, ‘not’, and theos, _‘god’), the view that there are no gods.” [ed. Audi (Cambridge, 1999), 59.] In the case of ‘amoral,’ ‘atypical,’ and ‘asymmetrical,’ the object being denied is not a belief but a thing. ‘Amoral’ is the lack of moral property, not a mere lack of belief in morality. ‘Atypical’ is the lack of something's being typical, not a lack of belief in what is typical. And ‘asymmetrical’ is a lack of symmetry, not a lack of belief in symmetry.
Several everyday people on the street understand “atheism” to denote a “lack of belief” in God.
There are dictionaries which include a definition of atheism that is synonymous with “lack of belief” in God.
This is relevant because if anything determines the meaning of a word (in the relevant context) it's the dictionary.
“INFORMAL: used to emphasize what you are saying:” ... “INFORMAL: simply or just: e.g. Then you literally cut the sausage down the middle.” Many atheists are using “atheism” the sloppy quasi-informal way that some people use “literally”, a way that should not show up in rigorous conversation. (Side note: Some dictionaries, like the Cambridge Dictionary above, won’t even give the courtesy of mentioning the informal imprecise usage of “atheism” as a lack of belief. These dictionaries think it’s not even used enough to even be descriptive informally yet. (The Cambridge Dictionary simply says, “the belief that God does not exist”).
Dennish Baron (Professor of English and Linguistics at the University of Illinois): “What dictionaries are, instead, are records of how some speakers and writers have used words. … This constant cherry-picking [between contradicting dictionaries] confirms that jurists do recognize dictionaries as fluid and context-bound, much like the words they define. The job of the lexicographer is not to give the law, or even to interpret it. Dictionaries don't exist to create meaning. Instead, they record the meanings assigned to words and phrases by speakers and writers, by professionals and amateurs, by lawyers and judges, by upright citizens and criminal defendants. These meanings are multiple and changeable, and reliance on dictionaries should always be instructive, never absolute.” visualthesaurus.com article