How is it wrong for all atheists and how do you define a belief? One (actually two) great atheist I knew before affirmed as true that it is certain that God does not exist, and not only that, she argued that it was impossible for God to exist. Is that not a belief? If not how do you define it [belief]?
First off a lack of belief is not a belief. For example I may not know anything at all about a subject. I may not even know that subject exists. I would have a lack of belief about that subject either way. This should be rather obvious.
Next there is a subject that I cannot have full information about. The rational path is not to accept a claim as true without sufficient evidence I do not believe people that make certain claims. That does not mean that I believe that they are wrong. They may be right, they may be wrong. I simply do not know enough to form an opinion either way. That is how most atheists are about the existence of gods. They lack a belief in them. That does not mean that they believe that they do not exist. That would take more information.
Now they may be able to refute the claims of some theists, or at the very least demonstrate that their beliefs are unjustified. For example when a theist makes the statement "I know that God exists". They put an enormous burden of proof upon themselves. Knowledge is demonstrable and if they cannot support that claim with clear evidence it becomes quickly apparent that all that person has is belief.
And yes, some atheists will gladly take on the burden of proof by claiming that "There is no God". I do not claim that. I simply state that I do not believe in god.
Let's take the Loch Ness Monster. There are many people that believe that they saw Nessie. They will even give examples of rather poor evidence for her. They are not that much different from theists that claim to know that God exists. I lack a belief in Nessie. I will say that she probably does not exist. But it is not the end of my world if she does turn out to exist. Do you know what it would take for me to change my mind? Just some clear evidence.
I will say almost the same thing about God. All it would take would be for some clear evidence for God and I would change my mind. The people that are being irrational are the theists that claim no amount of evidence would change their mind.