There are many people who don't believe in a god but who don't call themselves atheists, because they don't regard it as important. They call themselves libertarians, or motorcyclists, or poets, or whatever they are passionate about. People who CALL themselves atheists do so because they believe it is IMPORTANT to be free of religious belief. They believe that religion is not only false but also HARMFUL.
One big way that religion is harmful is that it teaches a false theory of ethics. The "divine command" theory says, in short, that obeying the alleged commands of a god is good and virtuous, and disobeying is bad. Just as religious faith is believing what you are told, so religious ethics is doing what you are told. Even if, Especially if, what you are told is bizarre and absurd.
This example, of evangelists teaching children to obey the alleged commands of a god always, even if the reported command is to commit genocidal war, is the clearest illustration of the harm that religion does.