I'm and atheist, and I'm proud of it. I go to a Christian/catholic school and I am one of the few atheists who attend.
One of my main reasons for being an atheist is simply because of the hurt that religion has wrought over the many many years of human history.
The ok'd testa eg is ripe with mass genocide, murder and all other kinds of sin, however, it is religion itself that speaks out against it. I kind this to be contradictory in the extreme.
When stating that I was an atheist for one of the first times, I was ridiculed tremendously because of an opinion and my way of thinking - by religious people.
People say that atheists are horrible people, bias, sinners, blasphemers, and other kinds of absurdity. However, they never stop and check they're own actions which are just as bad, if not worse.
Religion is the cause of most hurt, suffering and pain. To be 100% honest, I wholeheartedly believe that the world would be a far better place without it. To quote Stephen Hawking:
"science will win over religion in the end because it works."