Every now and again, I find myself talking to a religious person who declares that "you have to be pretty damn arrogant to be an atheist!"
To be clear on what they are saying, I always try to follow this up by asking whether it's that they have met arrogant atheists, or whether there's something about atheism that they believe leads people to be arrogant, etc. With very rare occasions, they come back with "it's arrogant to think that there isn't a god!"
Certainly, it's arrogant to say "I know that there is no god!" Humans aren't capable of knowing anything with absolute certainty, there's always the possibility of us being wrong. However, it's no more arrogant to say "I know that there is no god!" than to say "I know that there is a god!" And depending on the line of thought that resulted in this conclusion, there are more and less arrogant paths leading to either of those conclusions.
What's more, the admittedly arrogant statement "I know there is no god" is significantly less arrogant than "I know there's a god, AND I know that he wants X, Y, Z,and requires humans to do A,B, C." The more that is added on to what one claims to know with certainty, the more that the claimant is asserting that their own beliefs are supreme over anything that anyone else might state.
So, at worst, a "strong" atheist (one who claims to know that there is no god) is not inherently any more arrogant in their beliefs than most believers in a god or gods, and in many cases may be less so.
What's more, most atheists are "soft" atheists, like myself. I do not claim to know that there is no god, but I look at the world around me, and I see no compelling reason to believe that there is a god. Some people, people who are not me, will assert that I am an agnostic. That is only kind-of true, though. I don't claim to know whether or not a supernatural entity that might be called a god exists, true, but I do think that the existence of such a being is extremely unlikely. I am open to evidence that I am wrong, certainly, but after spending many years searching for such evidence, I have finally stopped pursuing threads that all lead to dead ends. The question of the existence or non-existence of gods occupies my thoughts only in so far as those who believe in the existence of such entities try to force me or others to accept their own (consistently unsubstantiated) claims*.
Is it still arrogant for me to say that "I don't think there's a god"? I don't believe that it is, but perhaps I am wrong. The reality is that every one of us thinks that we are right and correct in our beliefs, otherwise we wouldn't hold those beliefs. But the notion that my conclusion that it is unlikely that there is a god is somehow more arrogant than someone else's conclusion that there is? Well, that's an astoundingly stupid (and, let's face it, arrogant) notion that exists not because it has any merit, but because it allows people to focus on the alleged faults of others rather than turning inward and examining their own beliefs.
*A very common question from religious believers is "why don't you atheists leave religious people alone in their beliefs" to which my own response is "the majority of us would be happy to do so, if religious believers weren't busy trying to use the force of law (int he form of things like Proposition 8, DOMA, "blue laws" etc.) to force us to conform to their beliefs."
In other words, we'll stop bothering the religious if they'll stop bothering us. And yes, I am aware that many religious people do have a "live and let live" attitude, which is excellent, but: A) enough don't that these laws stay on the books or get voted into law, and B) the refusal of many of the moderate folks to speak up against the zealous and militant means that you allow the militants and zealouts to claim your name and speak for you, so I don't want to hear you complain when you get lumped in with them - if you don't stop them from claiming the name of your religion, then it's your own fault if you are considered to be like them. It may not be fair, but it is the way that society works. And, hey, I get lumped with with assholes like Christopher Hitchens, so it goes both ways.