An atheist is a person who believes that god does not exist. An agnostic is a person who does not believe that god exists, though agnostics often allow for the unprovability of a god. Though it seems like minor semantics, there really is a fair difference in that one is a belief while the other is a lack of a belief.
Faith is something that is believed, or as the Bible says, the substance of things hoped for, the evidence of things not seen, from the book of Hebrews in the New Testament. Christians often use as an example of faith sitting in a chair. According to the example, one has faith that a chair will hold their weight, but this is an inane example. One has every reason to believe a chair will hold them based on past performance of chairs, which is why it's so damn funny when someone sits in a chair that doesn't hold them but instead sends them ass over tea kettle. This isn't really the point here, but it is indicative of how faith can be seen as something that it isn't.
Some atheists are extremely dogmatic in their belief in the nonexistence of god. Dogmatic means to strongly hold opinion as fact. One could even suggest that these good people are atheist fundamentalists. Fundamentalists are people who strongly demand adherence to their belief system.
These fundamentalist atheists see the world in strongly black and white terms. To them, you are either an atheist with a strong disbelief in god and anything unprovable, and therefore rational, or you are irrational and may actually believe in gods, unicorns and farts that smell nice.
Atheism is a belief system, though many adherents would strongly disagree with this fact. I claim to be an atheist, and I may just mean weak atheist. I do believe that god does not exist in the form we've been taught generally to view him. I do believe that this stance is not at all provable or unprovable. Christians can no more prove their god does exist than atheist can prove that he does not exist. To me, this means that both views require faith. Don't tell the fundamentalist atheist this, as faith is completely anathema to them. Faith, to them is irrational, and they would not dare be irrational. Yet, as we've discussed, faith is a belief in something for which we have no proof.
Is it possible that there exists some broadly categorized interstellar entity with the power to have created the earth and the living being upon the earth? Nothing I've seen suggests that this does in fact exist, but I must also admit that nothing I've seen suggests that it isn't possible. I tend to doubt that it's likely, but I have no argument to stand up to the possibility of this. I would argue that god doesn't seem likely based on my view of the earth, and I'd further argue that the god I was taught about as a child seems highly unlikely to exist based on my understanding of the world. But however we dance around the argument, neither my stance nor that of the christians of the world are provable or unprovable.
That's what it comes back down to, proof. I don't think that there are any vampires, but if I were one, I'd not want my existence to become known as many people would likely want me dead, Anne Rice fans notwithstanding. Are there Martians? If so they too have done a great job of hiding, but again, the rover not seeing them isn't the same as the rover sending us proof that there are no Martians.
In the end, we come down to so many people are willing to strongly argue their personal convictions as fact. To do so is dogmatic fundamentalism, and I contend that this attitude is more ruinous to human relations than anything short of war. It is sad when self professed scientists argue so stridently against the existence of god when science should be expected to contain itself within the realms of things provable whether that proof is able to deny or confirm the existence. If it isn't testable, it isn't science.
Hat tip to PZ Myers since without his usually interesting blog, this subject would not have nestled in my brain as a grain of sand around which the pearl that is this post formed.