Is atheism a belief?

[Belacqua]

It's not a matter of belief. There's no real evidence to support the claims made in the Bible. 

Again, I am using the term "belief" in its simplest sense: something one holds to be true.

You have very specific criteria which you hold to be true concerning what constitutes good evidence. Other people disagree with you. When talking with such people, you would have to defend your beliefs concerning what constitutes good evidence. 

I'll guess (and you can correct me if I'm wrong) that when you say "evidence" you mean input of the type that scientists use. Religious people may also include revelation, authority, tradition, the logic of natural theology, and the logic of metaphysics. They may be wrong to do so. But the position you hold and the position they hold are both criteria by which you reach conclusions, and require support. 

I won't pretend to know what "every single adult atheist" thinks, but by definition, they do not have a belief in any god or gods.

By definition, they hold it to be true that the claims they have heard concerning god are unpersuasive. Do you have an argument against this? 

The entire concept of religion requires you to fit reality into your preconceived notions, not the other way around. 

That's an interesting assertion. Is it true in every single case? Are all religious people, of every type everywhere, operating in the way you say? 

Right now I'm reading the autobiography of a very intelligent and wise scholar. She studied biology at Cambridge in the 1930s, during the time when Russell and Wittgenstein's views of religion were at their most influential. None of the undergraduates in her circle took religion seriously, all of them thought that a purely materialist view of the world was the only truth and the only thing possible for honest people in the future. 

If you're into poetry at all, it will be meaningful for you to know that in those days Yeats was considered laughable while William Empson's poetry was seen as the wave of the future. 

Cambridge in the '30s pretty much pioneered the kind of beliefs that are popular on this forum.

Anyway, that scholar discovered that the purely rationalist view of things failed to explain the events of her life. After decades of conflict based on trying to retain this science-only view of things, she was reluctantly forced to change. She became an important scholar of Neoplatonic philosophy--including its religious aspects--and a wise spokesman for the value of such a view. To his credit, William Empson, who despised Christianity, continued to value her advice. 

So there is an example of a profoundly intelligent woman who doesn't fit your description of what religious people are like. And I think it's better to examine the ideas of intelligent people rather than stupid ones. 

because religious folks refuse to even define what god is.

This is downright false. This is something you believe which is blocking your ability to analyze things clearly. 

There are any number of reasons to fight with religious people. It's true that different views give different and incompatible definitions. It's true that many of the definitions are not persuasive. It's true that the older definitions are incompatible with good science. But to say they "refuse to even define" it is false. 

An example: actus purus. This is an important definition in the Christian tradition. It may be good and it may be terrible, but it is a definition.