Atheism
Ok, so you can basically see what Atheism is by just looking at the word. It is simply the absence of Theism, the belief in a god. You see the absence of Theism by the a- in front of it...an example of what that means is the word Asymmetrical. Take the A off...its symmetrical. Understand? Good.
Atheists simply just don't belive in any Gods or Goddesses. This doesn't mean that they are satanic or immoral. Just like Theists, their beliefs are highly personal and vary from person to person. Their beliefs make them no worse or better or wrong or right than any other person, despite what some highly intolerant, fundamentalists of other religions might like to say.
Now for a bit of historical information. There really aren't many ancient accounts of atheism. The fact being that many ancient civilizations used pictures to record information. It's much easier to draw a picture that represents Mr. God/Ms. Goddess creating the universe and the human species; than it is to draw a picture that shows that one doubts a god/dess' existence.
In the Middle Ages and Renaissance, it was hard to openly express atheisitc views without being branded a heretic and/or exectued by the Catholic Church. Among some of the humanists of that time that inspired future atheists were Leonardo Da Vinci (1452-1519), Desiderius Erasmus (1466-1536), Thomas More (1478-1535), Giordano Bruno (1548-1600), Galileo Galilei (1564-1642), and Johann Kepler (1571-1630). Many of these people were considered heretics and some were even executed for their beliefs.
In the 1700's a more open view on atheism was taken. While writing the constitution of the United States of America, Thomas Jefferson wrote the 6th article stating there shall be "no religious tests" for any public office or trust, and was instrumental in writing the 1st amendmant "Congress shall make no laws respecting an establishment of religion, or prohibiting the free exercise thereof." He also stated...
Thomas Paine, the man who write the first anti-slavery law of the U.S.A. wrote The Age of Reason . This book is extremely critical of Christians, and is popular amongst atheists.
This is a very abbreviated history of atheism, and it has many gaos in it. This is just to give you an idea of some of the founders of free thinking rights. If you want to know more, I would suggest looking up "Atheism" on a search engine.