Yeah, let’s try to get a few things straightened out regarding some definitions. I’ve noticed a lot of people seem confused regarding the definitions of atheism and agnosticism. As a bonus we’ll go over antitheism as well.
The root words here are theism and gnosticism. Theism deals with belief, or what you BELIEVE to be true. Gnosticism deals with knowledge, or what you KNOW to be true.
The prefix a- simply means “not”.
Theism is the claim that some deity or deities exist (henceforth referred to as just “god” for simplicity). Atheism literally means “not theism”. So “not belief that some god exists”. It is NOT saying “no god exists”.
Gnosticism, in the context of some god existing, is the claim of knowledge either way. Either “I know a god exists” or “I know no god exists”. Unless you have some evidence that no one else has, either of these claims are irrational. This evidence would have to be something conclusive and objective that you can show everyone else so they will know too. Or as AronRa says “If you can’t SHOW it, you don’t KNOW it”.
So the only rational position is agnosticism, meaning “not knowledge”, or just plain old “I don’t know”.
Let me be clear. Agnosticism is NOT the third option. EVERYONE is either theist or atheist. You either believe the claim or you do not.
On to the bonus round: antitheism. Again, the root word is “theism”, and you tack on the prefix “anti”, meaning “against”. So “against theism” or “opposed to theism”, theism being the belief in some god. Again, you can be theist or atheist and also be antitheist. I can’t put it much better than the antitheism article on Rationalwiki:
“Antitheism is the belief that theism and religion are harmful to society and people, and that if theistic beliefs were true, they would be undesirable.”
And again from the article:
“Whereas agnosticism and gnosticism address knowledge about gods, and whereas atheism and theism address one’s belief in the existence of gods, antitheism addresses the utility and favorability of theistic belief.”
So there you have it. Even if right now I found conclusive proof of a god existing, thereby becoming a gnostic theist, I would still be an antitheist, because I would still think that the mere existence of any god is undesirable.