Is atheism a belief?

[Belacqua]

The problem with that is, if you share the criteria such as evidence based and so forth with a theist, who also employs that reasoning in evaluation of claims, then it isn't the criteria themselves that are doing the work in separating the theist from the non-theist.  In order to meaningfully assert that atheism or non-theism were a belief, the beliefs in question must be both relevant and necessary to forming that conclusion.  In this case, one can hold the standards and not come to the same beliefs or lack thereof, so the belief in these criteria is accidental, not a necessary part of the identity.  Accidental properties do not define a thing.

That's good, yes.

The criteria themselves are not the distinguishing feature. Reasonable people may use similar criteria and reach different conclusions. 

Atheism is the conclusion, based on premises, that certain claims are unpersuasive. It is a belief (a thing held to be true) based on prior beliefs (things held to be reasonable criteria).

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The definition remains the lack of one thing.

Yes, but in adults who live in a society, that lack is a belief that certain claims are untrue, based on prior premises.