We are born atheist. At some point, religion is instilled on us. Usually, it's our parents that make this decision for us. If you regularly go to church until you are about 7, you will probably remain in that religion for life because you are too scared (hell or death threats) to look at alternatives. If you are weak in a church, don't go at all, or never took to the religion by then, chances are good that you will look elsewhere or remain atheist.
Another way atheists come along is when Christians (or other religions) look at their holy book, realize that it is impossible, and decide there is no God at all. Agnostics are those who merely doubt the existence of a God. This is a sign of lack of culture or when people reject the culture they are brought up with. Generally, they develop a secular culture--which is not all that bad. People that become atheists generally live their lives for their lives instead of living for when they die. This is a big step up from religions that prevent people from exploring other possibilities.
Then you have people that turn to Satan. Some do this because they were abused in their parent religions. Some do it because, when they analyze their "holy book", they see God as evil and Satan as just trying to liberate us from God's tyranny. Some get into the occult. A few actually worship evil--those are the ones that sacrifice children and live animals, drink blood, and terrorize people that go to churches. (Notably, even that group isn't as harmful to the whole of society as Christianity, which has retarded science and enslaved so many people, robbing them of cultures that did them good and replacing them with rubbish.)
Which is right? We don't have any absolute proof, but if Christi-SCAM-ity or Islam are right, that is a pretty bleak picture for us after death. Either torture by infinite light or torture by infinite heat. Left alone, science will eventually prove something right. Physics at the quantum level will prove that there is an astral plane where angels (filth) or demons (or both) exist, people who die go on to live there, and reincarnation is real. Or, it will prove it once for all time that it is all bunk. But, as long as religion is going to force scientists to throw out results that disagree with their own paradigm, we will never have absolute proof of anything. Until then, both atheism and Devil worship are plausible, while the Big Three monotheistic religions and any religion they have corrupted are busted.