I'm atheist since I was born. Why? Because it is impossible to ignore the fact that every person thinks their religion is right, and if each person is right, then they must all be wrong too.
Humans can't be created by God's spit, and risen from the ground at the same time. That is why I support science.
I have created my own religion, though
somewhat similar to Greeks/Native Americans. It is based on all these spirits that control nature and a few spirits of wisdom trying to balance the world against spirits of death, hunger,etc..I don't actually believe it, of course. I just created it for fun.
I always thought it was silly how people thought of themselves as a religion just because their parents were. There are some people who say they are Christian just because that is what their parents said they were, and because they go to Church once every two weeks. Yet only like 1% of "Christians" actually believe the stuff in the Bible. If you ask them if they thought that women came from Adam's rib, they would laugh it off.
I think that about 90% of people are actually atheists, but identify themselves as whatever religion their ancestors were.