Its takes just as much faith to try and prove their is no GOD than to prove there is a GOD. From my dealings with pronounced atheist most are mad at GOD about something. Friend got hurt, Mom died of cancer, mean people at church when they were kids. Hence their desire to prove GOD is not real and convert as many people as possible to their beliefs. IE their religion. There is a fair amount of other people for different reasons that claim to be an atheist but most don't put much effort into it. My experience is agnostics are much more common. These are people who never thought about it or don't want to think to deeply. Let them be and they will let you be and everyone will be happy. I would say those are really the only nonreligious group in the USA.