There's been a tendency among atheists to use a recent definition of the term, one from Anthony Flew.
This is presented as a legitimate intellectual position, it is "the absence of a belief in God".
This is preferred over the more traditional definition "the belief there is no God" which is the definition you'll find in most historic philosophical books on the subject.
The latter of course cannot be sustained by an evidence based argument (something atheists demand from theists all the time) so by using the first definition the atheist is safe, their position is (apparently) easy to defend "I've seen no evidence".
Their position (which they think requires no evidence) "the absence of belief in God" is thus presented as being in no need of proof, or support, it the "default" position they say, and all the pressure can be applied to the theist who must argue and defend their presence of belief, with evidence.
But I put it to the reader that this is just trickery, by replacing the term "believe" (which is a conviction some proposition is true) with the term "absence of belief" it is claimed no evidence is now required, only belief requires evidence, no evidence is required to "not hold a belief" - but is this true?
A predicate like "God does exist" is binary, it can only have two values - true and false, it certainly cannot be both.
Now just as I cannot say "I believe God exists and I believe God does not exist" I surely cannot say "I do not hold a belief that God exists and I do not hold a belief that God does not exist"?
How could one adopt such a position? the only way is to rephrase it as "I do not know if God exists" that is certainly possible, and that is the true default, agnostic.
But many ardent atheists refuse to be described as "agnostic", not for them the soft position, not for them the admission that God might, just might exist, oh no that will not do.
So the position they adopt is "I do not hold a belief in God" but do they not grasp? this is logically indistinguishable from "I do hold a belief in not God".
For if X is true or false then to not assume it is true is the same as to assume it is false!!
(e.g. if I base some outdoor planning decisions on the position I am not going to assume it will rain, then I must, unavoidably base my decisions on the position I am going to assume it will not rain).
It cannot be any other way - to not assume God does exist is a choice (for it is either true or false) just as much a choice as to not assume God does not exist.
