"Atheism" is often oddly defined as lacking a positive belief or opinion affirming the existence of any God. We define "atheist" in this way to distinguish ourselves from people who do have a positive believe affirming the existence of at least one God.
In this sense, atheism fails to rise even to a philosophical position. It's a purely lexicographical definition: it describes what all (or almost all) people who call themselves atheist have in common, and what distinguishes them from people who call themselves theists. We define it this way not to be perverse or to make atheism so vague as to be immune from criticism but rather because people who call themselves atheists have not heard of or do not understand, or are apathetic about many conceptions of God. For example, Alice, who calls herself an atheist, might positively believe that Yahweh and Allah do not exist, but she might have never heard of Liebniz' God, and she might not understand Spinoza's God. She cannot positively believe that Liebniz' God does not exist: she lacks any belief at all about Liebniz God.
People are atheists for a lot of different reasons. Some people's atheism — such as my ex-wife2's — results from a deeply considered investigation into philosophical and evidentiary arguments. Some people, such as myself, became atheists (or never became theists) for much more superficial reasons. I simply never became a theist because the whole idea of God seemed at a superficial level to be childish and ridiculous, and since an early age I was always discouraged from blind obedience. All of my own philosophical investigations into atheism and religion have come after my self-identification as an atheist. Some people are atheists for religious reasons: they have real religious faith in some system (such as Buddhism) that denies the existence of any Deity. Some are atheists for entirely dogmatic reasons: they believe everything their parents tell them, and their parents told them there is no God.
All of these people are atheists. We don't have a standards committee, we don't enforce any ideological conformity. If you don't have a positive belief that any God exists, and you want to call yourself an atheist, by golly you're in the club. We'll take everyone, even a gigantic asshole such as Stalin*.
*With all due respect to some of my friends, Stalin was a world leader, and in my investigations of history I've never encountered a leader of anything more complicated than a bagel shop who wasn't either a gigantic asshole or a pathetic idiot (and often both).
The point being that it's impossible to create broad generalizations about atheists. Indeed, since atheism is defined as a lack of beliefs, without specifying why that belief is lacking, you can't even draw general inferences. There's nothing at all about a person (other than their specific beliefs about a god) that would preclude them from being an atheist; there is no specific belief that is incompatible with atheism.
That does not mean, of course, that one cannot criticize atheism. There are uncontroversial subcategories of atheists — philosophers, scientific naturalists, materialists, social critics, anti-religious atheists — all with a fairly extensive literature subject to critique. If you want to criticize materialism, knock yourself out. But if you start your critique with a long-winded argument that all atheists are necessarily materialist, I've already written you off as an idiot before I get to your critique. (And when I do keep reading, I invariably find that the critique is so laden with fallacies and lies that my initial evaluation is never incorrect.)
In much the same sense, there are many uncontroversial subcategories of theistic religious belief, and it's equally unwarranted to draw many broad generalizations about theists. However, there is at least one broad generalization that one can draw about theists: they believe at least one proposition is objectively true that is — by naturalistic standards — either objectively false or not capable of being true or false*. If you are a thorough naturalist, it is impossible to be a theist, therefore we can at least infer that no theist is a thorough naturalist.
*If you want to talk about God as something other than a person, or you don't actually have a truth belief about God, then most atheists will probably consider your definition somewhat perverse and idiosyncratic but will usually exempt you from criticism... unless of course you're trying to use your idiosyncratic definition to make broad, unwarranted generalizations about other theists.
This asymmetry between atheism and theism means there is no "silver bullet" against atheism, but there is a "silver bullet" (thorough naturalism) against theism. In other words, the value of naturalism directly undermines theism, but undermining naturalism does not undermine atheism. If that seems unfair, too bad: that's the way it is.