The same goes for defining atheist, according to the OP. You either reject the proposition, or you don't. If you don't reject the proposition, you're not an atheist. All the reasoning in the world (whatever lead one to his or her rejection) isn't part of the definition.
That's not agnosticism.
But I don't reject the proposition! So how does that make me not an atheist? (!) And what isn't agnosticism?