When I was a believer, it wasn't very strong faith. I spend a hell of a lot more time as an atheist than I ever did thinking God existed. I've often wondered why this is, but I think I have a good explanation, at least for myself. To me, it's so obvious that God is made up and the stories of the bible are manipulative and exist to have people act a certain way. I don't know the purpose of the bible being manipulative, but I'm always amazed at how people buy into it. Teaching people to hate others for no readily good reason except for "because God says so" is destructive to society as a whole.
Perhaps the most glaring flaw I see in religion is that the universe is far, far bigger than most people give it credit for. The school of thought that everything was created with humans in mind, which is at the center of almost every religion, is obviously wrong when I look out on a starry night. By having religion put humans at the center of everything it automatically elevates them to being more important than they really are in the universe. Maybe it's just a result of people trying to find their place in the universe, but I'd rather face the cold hard truth than live in some "happy" fantasyland.
Those are the main reason I'm so interested in religion now that I'm an atheist: I don't understand how people can buy into it when it's so obviously not true.