By definition, an atheist is a person who does not believe in a supreme deity or deities. If I believe in the Hindu god Krishna only, that makes me atheistic towards Allah of Islam, the Flying Spaghetti Monster of Pastafarianism, and any other deity. The same works in any given situation. This, however, does not mean that we all should embrace atheism since it fails to answer one pretty big particular: why is there evil? In atheism, everything in this universe is all there is. But we have no standard for morality. It can't be subjective because people think differently from each other. But we can't look anywhere else because (according to the atheist) there IS nothing else outside of this universe (or maybe multi-verse if you're a Stephen Hawking follower).
The definition of atheism is:
"disbelief in the existence of a supreme being or beings." (dictionary.Com)
So, unless you believe in every supreme being that has ever existed and exists - yes, you are an atheist on some level.
In retrospect, it makes it quite amusing when people put down atheists and the atheist community. People are being (unknowingly) hypocritical.
It is undoubtedly true that the believer of one religion is in fact an "atheist" about all other religions in almost all circumstances. So in that respect, the answer is yes. But would most of these people allow themselves to see their isolated religion through this broad perspective? Doubtful. Would they be likely to admit they are on some level an atheist, despite the fact that they are by definition? Not bloody likely!
All theists are atheist in the sense that they don't subscribe to every god in the world except for the god or gods they subscribe to. For example Christians believe in Yahweh, but are atheist to Zeus, Dionysus, Horus, Rah, Krishna, Flying Spaghetti Monster, and (my personal favorite) Chalchiuhtlicue, etc. Obviously atheists go one step further and don't subscribe to any god.
As it has been stated by others all ready, we are all atheists in a sense. The Romans called early Christians atheists because they only believed in one god in comparison with the many gods. This was not a new concept, it had been around before the concept of Christianity. A bit of history: A man named Anaxagoras was sentenced to death around 450 years before the start of modern era when he had claimed that gods are just mystical abstractions.
As Professor Richard Dawkins put it:
“We are all atheists about most of the gods that humanity has ever believed in. Some of us just go one god further.”
Or a quote by Stephen Roberts (not sure who he actually is sorry)
"I contend that we are both atheists. I just believe in one fewer god than you do. When you understand why you dismiss all the other possible gods, you will understand why I dismiss yours"
You may disagree with these quotes and claim that atheism has a certain definition that renders my answer wrong. Fair enough. However there are many definitions of atheism.
It certainly it is a 'lack of belief' however some people claim that it is the outright claim that there is no god. If this were the case however then there would be hardly any atheists. Almost all atheists identify as agnostic atheists. Or at least claim they 'do not know if there is anything'. (meaning they are agnostics)
So whether you believe that I am wrong because atheists do not believe in ANY gods and you cannot be atheist to several or not surely you agree the quotes are pretty good.
There are literally thousands of Gods humanity no longer believes in, as Hitchens said, we are getting closer to the true number every time. Of course, my peronal preference is that of the great spaghetti monster, but Thor, Athena, Ba'al, Junos, Aphrodite, Brittania, these are no longer worshipped, of they are, then like all faith it is incompatible with an alternative.
If you ever find a complete Christian, Muslim or whatever without any doubt they are wrong ever, they just might be full of crap by being dishonest in favor of their own god. If one was to ponder, even miracles could be explained.
But atheist means they believe there is no god or any possibility of its existence, whereas agnostic means they aren't willing to move that far past the line of disbelief. Many theologians talk about the question "where did that come from?" or "why does that do what it does?" No matter what, in the cause and effect world we live in, those two questions will always appear. One could take that philosophy alone and never have a quantum of atheism within their heads.
You are correct on disbelief being in every believer, but not far enough to be from the influence of full on atheism.
(The question of where God came from is a different debate)
Because religion shouldn't factor into this question. I believe for instance we are all born atheists, we start out not believing and depending on what is instilled in us or from our own personal experiences, it really can go either way, is depending on what you believe is whatever religion derives from, is coming from you. There is doubt in every believer and non believer a like but it doesn't mean everyone is an atheist on some level, you can question something for instance but it doesn't make you not believe its not there. Questioning things is more along the lines of human nature, nothing to do with religion but more human concept on finding every known possible thing in the universe.