Atheism is the lack of a belief in god(s) and the supernatural.
That’s it. Seriously. There is nothing more too it.
A few common misconceptions:
There are no “doctrines” of atheism. There are no “leaders” or “popes” of atheism. Sure there are prominent speakers, scientists, and authors, but none of us take marching orders from them. On the whole we’re very independent. (This independence has historically hindered us in the fight for secularism in politics)
Atheists, while primarily liberal, can come from all political backgrounds. A prominent atheist, Christopher Hitchens, is a neo-con and supports Bush style foreign policy and the wars in Iraq and Afghanistan. We really do come from every walk of life. Because of this diversity and lack of organization, it is not unusual for us to disagree with one another.
As cool as it would be, we are not running a global conspiracy and we do not have an awesome secret lava base. (see above point):
The “a” in atheism should never be capitalized unless at the beginning of a sentence because we are not a religion.
Atheism does not make any claims. Saying “I know there is no god” would be intellectually dishonest. There is no way you can know. The correct statement is “Based upon the available evidence, I am unconvinced that a god exists.” This is perfectly fine because atheism, just like the justice system and science, operates off of the null-hypothesis. We assume a claim is false until it is proven beyond a reasonable doubt to be true. “Innocent until proven guilty” is a common example of the null-hypothesis. Theists make the extraordinary claim “My god(s) exist, they have these properties/characteristics, they’ve made everything, and actively intervene in my life.” We simply ask for extraordinary evidence to back up this extraordinary claim. Until that required evidence is presented, we assume the claim is false. Because atheism operates this way, many of us have a fondness for science which operates very similarly and is the best tool humans have for learning about and manipulating the world around us.
It does not take “faith” to be an atheist. Since atheism does not make claims and focuses on evidence (see above) there is nothing in atheism that requires faith (belief without evidence).
Atheism and agnosticism are different answers to different questions. They are not mutually exclusive. Most atheists, at least the intellectually honest ones, are agnostic atheists.
Atheists do not believe in Satan or worship Satan. There is no evidence he exists, just as there is no evidence god exists.
Atheists do not believe they themselves are god. As hard as it is for people to imagine, we don’t believe in the concept of god. Period. Not other gods, and not ourselves as gods.
Everyone is born an atheist. You only learn about the concept of god later in life, usually through some kind of indoctrination as a child.
We do not “hate god” or secretly know he exists but deny it because we wish to live a sinful lifestyle.
We are perfectly normal, moral, happy people capable of leading rewarding and fulfilling lives.