I am an atheist because I reject the idea that gods exist, in the same way and for the same reasons that I reject the ideas that that the earth is balanced on the back of a sea turtle, that homeopathy is more useful than a heart transplant, that Rapunzel wove her hair into a ladder or Rumpelstiltskin wove straw into gold, that stepping on a pavement crack will break my mother’s back, that a deposed Nigerian prince wants to email me several million dollars, that Uri Geller can bend spoons with his mind, that I am in danger from vampires or zombies or broken mirrors, or that I am protected by angels or leprechauns or horseshoes.
Reasons to Believe
Like many people, you may sincerely accept some of the above ideas as being true, either because you have experienced something unexplainable that has caused your brain to generate a belief in your God or Uri Geller, or because you feel happier when you believe in heaven or homeopathy, or because you prefer your life to be guided by holy writings or horoscope readings, or because you think that people behave better when they are being scrutinised by Satan or Santa, or because life is just simpler when you seem to believe what most people seem to believe.
Absence of Evidence
However, I reject all of these ideas simply because there is no evidence that any of them are true. Of course, I might be wrong about any or all of them. And I will happily change my mind if I ever get evidence that an alien spacecraft crashed at Roswell in 1947 and that successive US Governments since then have been hiding the aliens at a military base near Groom Lake in Nevada, or that the creator of the universe visited one small planet and caused a virgin of one species to give birth to himself so that he could die, return to life, and then write his story in a book.
Atheism is a Way of Thinking
I think I am very unlikely to get such evidence, because the ideas are so improbable, but if I get it I will not resist it. And this is the key to understanding atheism. It is far more about a way of thinking than it is about the outcomes that result from that thinking. Atheism involves believing in the power of reason to pursue knowledge, accepting ideas because of evidence, rejecting ideas because of lack of evidence, and always being prepared to change your mind if you learn something new.