In logic, either A or not-A covers all instances. Thus one would think that either theism or a-theism similarly covers all instances. But it does not. Between theism and atheism, as these words are commonly used, there exist a significant range of beliefs, in particular what might be called pantheistic beliefs in the broad sense of that term. Taoism and Zen Buddhism are two particularly interesting instances.
I would suggest that if we want a line that cuts across all instances, it would be the line between those who believe the universe is entirely an accident and lacking in purpose, meaning or any reason for being, and those who believe that there is a reason why the universe is here and by extension why we are here, even if we are not able to discern the reason.
There are those who believe that science supports the first view, but this is not correct. Science has demonstrated quite conclusively that the universe is lawful, and it has come to have a fairly comprehensive grasp of the nature of these laws, at least as they apply to simple physical and biological processes. But when it comes to the question “How is it that the universe is lawful?” science has little to say. There are, of course, theories, but none of the theories are either provable or falsifiable, and thus not genuinely scientific.**
Since we would have to understand how the universe came to be lawful before we could have any certainty about whether there was or was not a reason creatures such as we exist, any judgment on this question will be made with inadequate information. From where we stand it is just a mystery; agnosticism on this question is, therefore, the only position aligned with the facts.
** Physicist Lee Smolin argues that his theory of Cosmological Natural Selection, which purports to explain the lawfulness of our universe, is falsifiable and I see no reason to doubt his claim. But he still has to posit an original something that has the potential to evolve; Cosmological Natural Selection does nothing to explain the existence of that original something or its nearly infinite potential to expand and evolve.