You have actually a few concepts here, let's see if I can tease these out here or not.
We have a concept that no-one BELIEVES absolutely everything they hear.
So, if some hear that little green men invaded this planet just last year,
or if someone heard that a demon was born to a woman, then we think
they won't believe this.
And even if some DO believe this, they will then reject OTHER things,
is that the idea here?
I think it is, and I think so, too.
This is where the next concept becomes blurry for me, maybe
it's just me, but the meaning (as others said) of
atheist is ....a-theist...or without or away from or rejection of gods, all gods.
Someone who does not believe in Martians landing could be a Christian.
Someone who does not believe in a demon being born to a woman
would be most people, but those people might be
catholic or Muslim, or Buddhist.
So, is everyone an 'atheist' to someone?
Does everyone believe in 'no gods' re: someone else?
Some gods, yes, even most gods, yes. A Christian will
believe in the Godhead of the bible, but none of the other gods.
In that specific area, they would be 'atheist' re: the other gods
but theist re: Christianity. So atheist could not be used universally.
The cliche ...believes everything they hear often describes people
who can go any way at all.
Have you met people who go to this meeting...and believe it,
then they go to another meeting...altogether different and
tell you about that, and believe that.
Try telling them that the first meeting doesn't line up!!
These people have no foundation at all, and just float around,
but they do exist, but whatever they believe will soon be
supplanted by the next thing that comes along.
I think what you've put forward is an important concept
about belief here.