I wonder what would happen if agnostic was an option?
I'm glad it isn't because being agnostic has been taken by many to be some kind of third way between theism and atheism. Being Agnostic is beside the point of being a theist or an atheist, because you will inevitably fall into one or the other. Gnosticism/Agnosticism isn't a basis that constitutes its own category. For instance, I've never seen someone claim they are "Gnostic", ever. It wouldn't make sense, but people think just being agnostic does. What knowledge are you admitting to lacking? Generally everyone falls into these categories whether they like it or not: