Almost nothing illustrates the psychological phenomenon known as projection more vividly that the ubiquitous theistic pronouncement that atheists are arrogant.
In theory, atheists are arrogant because they have the temerity to suggest that revealed knowledge is less reliable than knowledge earned through observation, calculation, measurement and experimentation. The temerity to suggest that perhaps ancient collections of mythology and folklore may not be the most efficacious method by which to seek understanding about the Universe, the origins of life and the human condition.
In practice, however, theists proclaim that atheists are arrogant to conceal the mind blowing narcissism, arrogance, egocentrism and maddening anthropocentrism that lies at the heart of their cherished mythology. Theists proclaim the arrogance of atheists not because of the substance of our arguments, but because of the simple fact of our existence, because ad homenim attacks are the only way they can deal with the fact that the entire Universe makes a mockery of their own arrogant certitude. It is projection made manifest.
To illustrate, let's examine a few comparative statements of belief and see which exhibits greater arrogance.
Theist - I KNOW God Exists.
Atheist - I do not believe that God exists but do not believe that it can be definitively proven either way.
Theist - I KNOW God is aware of our existence and that we are important to Him.
Atheist - Even assuming there is a God, I see no evidence that it is aware of our existence or deems us to be important or interesting in any way. Given the size and scope of the Universe I doubt that such an entity would be the slightest bit interested in creatures as finite and cosmically insignificant as we are.
Theist - I KNOW God has appeared to humans.
Atheist - As there is no evidence for such an occurrence I doubt such an appearance ever occurred.
Theist - I KNOW God revealed knowledge to us and that the Bible/Quran/Book of Mormon/Dianetics etc. is true.
Atheist - There have been somewhere on the order of 2780 different deities throughout recorded human history, nearly all of which had manuals of some sort, nearly all of which are expressly mutually exclusive and nearly all of which are internally inconsistent and contradictory. I see no reason whatsoever to engage in revelatory preference and choose one internally inconsistent mythology over another.
Theist - I KNOW God's personality.
Atheist - Assuming the existence of God, I would not be presumptuous enough to claim to know or understand anything about its goals, desires, motivations or otherwise, let alone claim to know that humans are somehow relevant to said motivations or are privy them.
Theist - I KNOW God desires a personal relationship with me.
Atheist - Assuming the existence of God, I see no evidence whatsoever that such a being would be the slightest bit interested in our entire galaxy, let alone our humble star or our infinitesimally tiny planet, let alone that such a being would desire a personal relationship with an individual denizen of such a world. To make such a claim is the equivalent of a subatomic particle claiming that I desire a personal desire with it.
Theist - I KNOW God answers my prayers.
Atheist - I see no evidence whatsoever that God answers prayers any more than a milk jug answers prayers.
Theist - I KNOW all other God Beliefs are False.
Atheist - I believe that the supernatural claims and hypotheses offered by every religion can be readily explained through rational means that do not require the invocation of any divinity or can be dismissed due to conflicting, contradictory or simply non-existent evidence. As these supernatural claims go wholly unsupported when subjected to rational inquiry, I believe that all such supernatural claims are false regardless of which religion posits them.
Theist - I KNOW that when my Faith is contradicted, my Faith is correct regardless of any evidence to the contrary.
Atheist - Expressions of belief in an unrealized proposition despite evidence that said proposition is false is illogical and irrational.
Theist - I KNOW atheists are arrogant.
Atheist - Theists who claim not only to KNOW that God exists, but claim to KNOW his mind, his character, his desires, who claim that humanity is somehow special and privileged in a Universe filled with hundreds of billions of galaxies and hundreds of sextillions of stars, and septillions of planets, that such a being has chosen to have a personal relationship with them as individuals and that their chosen deity will punish those who disagree with them and their chosen mythological book for all eternity think we're arrogant? That's rich.
Sorry theists, but most of our lives aren't even interesting enough to justify an hour of reality television a week. The idea that the Creator of the Entire Freaking Universe, Sovereign over hundreds of billions of galaxies, sextillions of stars and septillions of planets is fascinated by the mundane trivialities of our lives - THAT is the epitome of narcissism and egotism. It is arrogance that borders on madness. You proclaim us arrogant to disguise your own.