(I wrote this as a companion piece to a longer, and admittedly somewhat rambling and abstruse, essay My Worldview)

What does it mean to be a believer or a non-believer? If religion is a matter of faith, that begs the question - faith in what?

On matters of belief, there are only two basic possibilities:

Atheism - Lack of belief in any deity

Theism - Any belief in a deity; includes a great variety of opinion or understanding, such as mono-theism, poly-theism, pan-theism, deism*, etc.

On matters of religious knowledge, there are at least five positions I know of:

Anti-theism - The position that a deity/deities do not or cannot exist; the position that the existence of a divine being can be disproved (sometimes called “strong” or “positive atheism”).

Agnosticism - Literally, no knowledge of God; the position that a deity/deities (or the supernatural in general) cannot be either rationally proved nor disproved. Sometimes this is called “weak” or “negative atheism.”

Agnosticism allows for the possibility of the existence of a deity/deities, but is skeptical of it. In colloquial reference, an agnostic is someone who is not sure what they believe, but doubts that there is a God unless they find direct evidence or have some personal revelation.

Gnosis - In Greek this is firsthand knowledge of a deity/deities. Gnosis is a conclusion arrived at through experience rather than intellect (via intuition or the senses, e.g. having a vision of God). Gnosis can be a personal experience (me and the deity meet) or trans-personal (a peak experience outside an egoic frame of reference).

Theory - In Greek “theoria” refers to rational speculation, also called “dianoia” and “dialectic.” In religion this is an understanding of a deity/deities through intellectual means (except in Eastern Orthodox Christianity, in which theoria refers to divine experience and is more like gnosis). Theory is important in theology, which is an academic field that attempts to logically study or prove the existence/nature of God (not to be confused with philosophy of religion, where a starting premise such as “God exists” would itself need to be qualified). Theoretical theology is often grounded in sacred scripture, but provides details and commentaries that go beyond a literal reading of scripture.

Faith - The position that a deity/deities exist(s), one which does not depend upon proof grounded in theory or gnosis; a strong belief justifies certainty. Faith is usually based on text-based creeds, assertions from religious authorities, and an emotional conviction.

I occupy the seemingly impossible position of being both an atheist and theist. This needs some explaining!

With respect to belief in a deity/deities as a specific being(s) or substance, I’m an atheist. 

But there’s what Frithjof Schuon called “Beyond-Being.” As Heidegger and Tillich explained it, Being itself is not a being, but the essence of being(s). 

In Hermeticism, this is called “the All.” The All is not a personal God who lives in the universe. Nor is the All a master deity who exists independent of time-space; one who creates the universe, wills it into existence, and controls it. It’s the universe itself, as well as the sense of the numinous.

This excludes just about every other version of theism that I know of. If deities exist in any sense, they are symbols of the Divine – signifiers and memes – not substantial entities.

When it comes to means of knowledge, the categories are not as exclusive as that of belief. For example, one can be an atheist agnostic, or a monotheist agnostic. Thus, it’s not that one can only be, say, a monotheist with faith or atheist. A belief can be based several factors.

What is my belief based upon? To put it another way, what approach to religious belief do I take? Agnosticism does not quite fit, although supernatural beings cannot actually be disproved (there's simply a lack of good evidence for purported miracles and magical beings in our universe). I agree with non-theistic arguments from philosophy and science, I theorize based on my own experiences, and I have faith that my positions are correct, even though I know I could be wrong.                                                                                           

* Deism is technically not the same as Theism in the sense that it is not belief in a personal God. However, it does hold that at one point a deity was involved in setting the universe into motion, so it cannot be classified as a form atheism, unless by this one means denial of a personal God that still plays an active role in the manifest realm. A personal God is one that we can have a meaningful relationship with. Deism, on the other hand, is only belief in God as a logical necessity (as in Aristotle's Unmoved Mover) but does not contradict the natural laws of the universe as they currently operate.

Further Reading: http://atheism.about.com