By definition, an Agnostic knows nothing.
Are you searching Dictionary.com to prove me wrong? The word agnostic, and many other words in the wonderfully corrupted language we call English, has had its definition warped by colloquial and idiomatic influences. I, being a lexical purist, vehemently eschew such distracting cultural movements and adhere to the true definition of agnostic. And by this true definition, an Agnostic does not know anything.
Let's take agnostic back to its roots to ascertain its true meaning. We see it has the prefix a-, which means "without", and also the Greek gnosis, which means "knowledge". With this we see that agnostic means "without knowledge", so an Agnostic truly does know nothing. More specifically, an Agnostic makes no claims to having knowledge.
Take a look at this wonderful chart I found laying around on Wikipedia (do not worry. I referenced the article from which I took this image, and that article is indeed accurate, despite Wikipedia's infamous reputation):
At the right side of the image, we see a bar which represents one's claim to knowledge. At the bottom, there is a bar representing one's strength of belief. The top of the Knowledge bar indicates no claim to knowledge, and the far right of the Belief bar indicates no belief. Thus, you can infer from this diagram that an Atheist has no belief in a god, but the amount of knowledge the Atheist claims to have in this regard can vary greatly. A Theist has belief in a god, but again, the amount of knowledge claimed could vary.
This is where Gnosticism and Agnosticism come in to play. These two philosophical methods determine certainty. For example, and Agnostic Atheist (which is what most people think of when someone says they are "an Agnostic") has no belief in a god, but does not claim with any certainty to know whether any sort of deity does or does not exist. Contrariwise, a Gnostic Theist has a belief in a god, and claims to know for certain that there is one.
Whether or not the person's beliefs are justified and indeed factual are another story entirely. I shall save that for another blog post. For now, let's remove the theology from Agnosticism and focus on the philosophy.
"I don't know and I can't know."
That statement, dear reader, is Agnosticism. It has absolutely nothing to do with the question of the existence of a deity. It is instead a question of whether or not anything can be known. I find this interesting, because if Agnosticism is justifiable, then by the logic of agnostic thinking, an Agnostic is by definition incapable of knowing that they are agnostic. Therefore, anyone who calls themselves an Agnostic is either mistaken or lying! With the understanding that agnosticism has had its meaning corrupted over the years, we find that most who identify themselves as "agnostic" are simply mistaken.
Everyone has doubts, naturally. In this way, everyone at some point has a measure of agnosticism. We can know some things for certain—as RenĂ© Descartes deduced: "I think; therefore, I am." Other things are rather uncertain, such as the origin of the Universe.
If you believe something, you must declare at least some agnosticism. One does not know beliefs; one believes beliefs. That is exactly why beliefs are called "beliefs": they are not factual! For this reason, we see Agnostic Theists, who say that while they believe that there is at least one deity, they cannot say this with any certainty, and so they simply have faith that their belief is justified. I personally do not have any doubt that there is a god (at least, not now!), but I do acknowledge that I could easily be wrong, so I must be an Agnostic Theist.
In conclusion, if a person identifies as an "Agnostic", it is safe to say that they probably mean "Agnostic Atheist". Do you see why definitions are important?