The hypotheses that Gandalf is a fictional creation has enough evidence to it that I believe it has risen to the level of a theory. Well, I'm being a little flippant here, but just demonstrating my point.
Same thing with unicorns, there is such a lack of evidence for the hypothesis of their physical existence that at this point it's been pretty much disproven.
The way God is generally hypothesized, it's hard (if not impossible) to prove or disprove. I don't adhere to someone's hypothesis that there is a god because they have not provided evidence that it exists. And maybe it's not even a valid hypotheses because it can't be proven or disproven. So in that sense I just lack a belief in their hypothesis. Making me an atheist. But to also hypothesize that you know for sure God does not exist is also equally, and in the same way, invalid.