Atheism is not a religion, but more a philosophy. It is the lack of belief in the existence of God or Gods.
Atheists have analyzed the available material evidence of the existence of one or more deities and has concluded that there is no real evidence of any Gods or Goddesses. They believe that the universe, Earth and its life probably evolved by perfectly natural processes, without intervention by a supernatural entity.
They generally feel that ethical and moral systems governing human behavior can be developed without reference to any code of behavior of allegedly divine origin. Other Atheists have simply never been exposed to belief in a deity or deities and therefor have no belief in them.
"Although the time of death is approaching me, I am not afraid of dying and going to Hell or (what would be considerably worse) going to the popularized version of Heaven. I expect death to be nothingness and, for removing me from all possible fears of death, I am thankful to atheism." [Isaac Asimov, "On Religiosity", Free Inquiry]
"And it is in his own image, let us remember, that Man creates God." [H. Havelock Ellis]