I think atheists believe in something that is undefinable.
Its okay to be agnostic, but I don't know what atheists believe other than what they don't believe, which is a cowardly way to rebel. It basically it means I'm undecided on a religion and don't know what I want.
You seem to think that being atheist and agnostic are somehow mutually exclusive things. This is erroneous.
Atheism is opposed to theism - these two things have to do with belief.
Agnosticism is opposed to gnosticism - these two things have to do with knowledge.
An agnostic atheist
lacks a belief in any gods but does not claim to
know for certain whether or not any gods exist. A gnostic atheist both lacks a belief in gods and goes so far as to claim that they do know for certain that no gods exist. (In a similar vein you can have agnostic theists and gnostic theists) Most self-described atheists I've ever had any form of communication with have been the former as I myself also am. Most self-described agnostics are atheists as well. The one and only requirement to be an atheist is to not believe any god claims. There is no dogma, no official meeting places that we're required to go to on specific days of the week, no "atheist Pope" or other central authority figure. Atheism by definition is only a stance on one type of claim.
I also take issue with your assertion that atheists take their position in order to rebel - this claim is asinine. The vast majority of modern atheists lack a belief in gods because those claiming that they exist have not met the burden of proof for their claims. That is not rebellion, that is called being rational.