Global atheism — that is, the belief that no gods exist — is no more problematic to justify than local atheism, which is merely the denial of a particular conception of God. Any failures in justifying global atheism are likely due to a person’s lack of understanding the difference between the two. It’s not that one or the other is justifiable more readily. There are several very good, simple, understandable ways to do it and I’ll maybe do two here.
First is a type of A.J. Ayer practicalism found in Language, Truth, And Logic, in that if God is said to exist yet only manifests in reality identically as any other phenomenon in reality, then no separate conception of God exists and so God (as people using the term unfailingly mean “other than nature” in some respect) doesn’t exist; nature does and God is simply another word for nature. If one takes the metaphysical challenge of God and “supernature”, then we follow George Smith’s epistemic problem onboard as articulated in Atheism: The Case Against God. That is, we have no possible conception of deity, any deity that is “beyond”. So then dialectically, if one supposes a supernatural God exists and if the reasons suggesting its existence are again natural, then God can only be an ill-conceived idea; we can’t know what the idea of God actually is except for “nature” or in other words, “reality” and in that case, there’s no telling about any God and every God can be rejected. The rejection is either justified via clear equivocation or because “God” and “No God” entail phenomenally to the very same natural world one only supposes would be different based on the existence of this ill-conceived synonym.
Second is far too simple. Functionally, there are only conceptions of deity and there can be no evidence for deity. Given that God manifests in reality indistinguishably from “nothing” if imminent, and given that only logical descriptions define what the nature of diety are, and given that logic doesn’t entail truth or tell us about anything at all, the statement that “Gods do not exist” is justified because of deflation. That is, any possible conception of God entails to a claim of ambiguity (ie. “otherness” which is incomprehensible) and equivocality (“sameness”, yet only through synonymnity or pure reasoning rather than observation). So, “Gods do not exist” arises from paying attention to the history of god-concepts and the justified expectation that there are no other possible ways to assert God. In other words, the success of global atheism entails the clear justification of local atheism, and even better, in an historical rather than merely immediate sense of god-talk. Even more simply put, the atheist who believes there are no Gods simply has the belief that no god-concept is novel or could be and history bears this out. This sort of atheist isn’t required to have his own concept of God to deny. In essence, there is genuinely only local atheism.
For those interested in my references but not so much as to want to look them up:
For if the existence of such a god were probable, then the proposition that he existed would be an empirical hypothesis. And in that case it would be possible to deduce from it, and other empirical hypotheses, certain experiential propositions which were not deducible from those other hypotheses alone. But in fact this is not possible. It is sometimes claimed, indeed, that the existence of a certain sort of regularity in nature constitutes sufficient evidence for the existence of a god. But if the sentence “God exists” entails to more than that certain types of phenomena occur in certain sequences, then to assert the existence of a god will be simply equivalent to asserting that there is the requisite regularity in nature; and no religious man would admit that this was all he intended to assert in asserting the existence of a god. He would say that in talking about God, he was talking about a transcendent being who might be known through certain empirical manifestations, but certainly could not be defined in terms of those manifestations. But in that case the term “god” is a metaphysical term. And if “god” is a metaphysical term, then it cannot be even probable that a god exists. For to say that “God exists” is to make a metaphysical utterance which cannot be either true or false. And by the same criterion, no sentence which purports to describe the nature of a transcendent god can possess any literal significance.
(A. J. Ayer, ‘Language, Truth, And Logic’, Dover, Second Edition, 1952, pg. 117)
To exist beyond the sphere of natural law means to exist beyond the scope of human knowledge; epistemological transcendence is a corollary of ‘supernaturalness’. If a god is a natural being, if his actions can be explained in terms of normal causal relationships, then he is a knowable creature. Conversely, if god can be known, he cannot be supernatural. Without mystery, without some element of the incomprehensible, a being cannot be supernatural – and to designate a being as supernatural is to imply that this being transcends human knowledge. Epistemological transcendence is perhaps the only common denominator among all usages of the term “god,” including those of Tillich, Robinson and other modern theologians. While some “theists” reject the notion of a supernatural being in a metaphysical sense, it seems that every self-proclaimed theist – regardless of his particular use of the term “god” – agrees that a god is mysterious, unfathomable or in someway beyond man’s comprehension. The idea of the “unknowable” is the universal element linking together the various concepts of god, which suggests that this is the most critical aspect of theistic belief. The belief in an unknowable being is the central tenet of theism, and it constitutes the major point of controversy between theism and critical atheism.
(George Smith, ‘Atheism: The Case Against God’, 1973)