If a person doesn't believe in god that doesn't necessarily mean the person believes there isn't a god. You're limiting the possibilities when there are others. One possibility is a person who hasn't formed any conclusion as to how the universe came into existence and because the person is unconvinced by theistic arguments the person hasn't granted the theistic proposition the status of truth in the person's mind. To not accept the theistic proposition as true is to disbelieve in god, but as I said it doesn't necessarily imply that the person believes there isn't a god.

Properly understood, theism and atheism are exhaustive terms like other sets of words like typical and atypical, symmetrical and asymmetrical, sexual and asexual, and so on. Whatever is having such terms being applied to it either fits in one category or it automatically sifts into the other. For example, if a day doesn't fit in the category of typical then it automatically sifts into the atypical category and if a painting doesn't fit in the category of symmetrical then it automatically sifts into the asymmetrical category. The words theism and atheism work the same way. If a person's theological beliefs don't fit into the category of theism then they are automatically sifted into the atheism category.

Agnosticism should not be understood as you're presenting it. By injecting agnosticism in this belief spectrum, the words theism and atheism cease being exhaustive. Linguistically, that makes no sense at all. Atheism simply means "not theism." Any "third option" would be contradictory--there's no way something could be neither theism nor not-theism. Agnosticism doesn't belong on this belief spectrum. Agnosticism belongs on the knowledge spectrum.

Agnosticism is linguistically similar to atheism, atypical, asymmetrical, etc. because it is also a word made by combining another word with "not." Agnosticism simply means "not gnosticism." The way gnosticism was used by Huxley--the person who coined the term agnosticism--was much more broad than the Christian sect known as the Gnostics, he meant it as "a person who thinks he or she has actual knowledge on the subject." An agnostic, then, is simply "a person who does not think he or she has actual knowledge on the subject."

Theism and atheism are on the belief spectrum while gnosticism and agnosticism are on the knowledge spectrum. With this in mind, one can draw a line from theism in the belief spectrum to the gnosticism of the knowledge spectrum, deriving the gnostic-theist position. You can also draw a line to agnosticism, deriving the agnostic-theist position. The gnostic theist thinks he knows God exists and the agnostic theist thinks God exists but doesn't think he or she has actual knowledge on the subject. We can also draw lines from atheism in the belief spectrum down to gnosticism or agnosticism on the knowledge spectrum. The gnostic atheist thinks he knows God does not exist and the agnostic atheist thinks God doesn't exist but doesn't think he or she has actual knowledge on the subject.

There is no way to be an agnostic without also being a theist or atheist. Similarly, there is no way to be gnostic without also being a theist or atheist. Regardless of whether you're gnostic or agnostic, you're still either a theist or an atheist.

Some people try to make a distinction between atheist and nontheist. This doesn't make any sense. The prefixes "a-" and "non-" mean the same thing. If two things which mean the same thing are attached to two other words that mean the same thing (theism and theism), the definitions for those two combinations will be the same. Arguing that "a-" and "non-" mean different things would lead to linguistic chaos and logical incoherence for a large portion of English. We would be haunted by ridiculous debates about whether a day is atypical or nontypical, whether a painting is asymmetrical or nonsymmetrical, whether we should call a certain skeptic an agnostic or nongnostic, and so on. Atheism and nontheism mean the same thing.

Atheism in and of itself cannot be considered a religion by any sensible definition of the word religion. A religion is essentially a worldview that typically contains notions of the divine as actual existing entities, rituals, liturgy, organized worship, priesthood, ethical codes, notions of virtue, and a set of myths or sacred truths held in reverence or believed by adherents. (Defining characteristics taken from Wikipedia article on Religion.) Atheism simply does not meet these criterion so it should not be called a religion. The notion doesn't make any sense. The notion makes even less sense when applied to the strong atheists, you're intended target. If you want to say Theravada Buddhism, Jainism, etc. (which are atheistic) religions then by all means do so, but don't apply the term to negative or positive atheism because it doesn't make sense to apply the term. In an attempt to try making negative or positive atheism look bad by stamping "religion" on it, you're merely debauching language.