I've wondered about this for a long time, especially as I myself once believed in
God too. At one short period of my life I was actually quite
religious (in the
Jewish faith), telling my mom not to switch on the light on
Sabbath and what not. But before I begin, a small point:
Whether or not God exists is totally irrelevant to this argument.
The first thing that sparked this thought in me (why people believe in God) is a book by Sigmund Freud, I'm pretty sure it was Moses and Monotheism, which outlines his theory of the evolution of the Jewish God. He was the first person, for me, who took a different look on the belief in God; not just "I believe in God" or "I don't believe in God", but "Why does one believe in God", at least this specific God. Freud talked about why God has the traits that we attribute to him today, and how belief has changed to give us the God we now have in the world. For that discussion, read the book. I want to talk about something different. This is my theory:
Take a kitten when it is born and keep it in total darkness, except for one hour a day, when you put it in a barrel with nothing but horizontal stripes. Do this for three months. Then take the cat out into the real world. After a few hours it seems almost normal. Only it can't see vertical lines. And it will never be able to see vertical lines. Does that mean they are not there? Hold a stick vertically, and move it sideways. The cat will not follow it. The cat will jump in pain when the vertical stick hits its nose, though. This experiment was actually performed with the above results (Hirsch & Spinelli, 1970).
There are many examples of how conditioning, especially at a young age, can affect an animal for good. So take a child, and tell him that God exists, and that he must fast on Yom Kippur if he wants God to forgive him, and that he must wear a kippah as a head cover from God, etc. etc. Do this for 20 years. What do you think he'll believe in afterwards? For many years, I believed that swallowing snot makes it go to your brain. I think it was only about 7 years ago that I said "Wait a minute, they lied to me!" I think I was told that once, when I was about 6. Well, I had quite hard facts at my disposal, and yet it took me a long time to figure out that there is no direct possible route mouth->brain. It's very difficult to find facts that refute the existence of God. In fact, as it is impossible to prove His existence, it is impossible to disprove it.
So here we have it. A simple case of classical conditioning. Take me, for example. I am quite a devout atheist. On Yom Kippur, I naturally don't fast, but I still feel a tinge of guilt every year. I had to wonder where that comes from. It's so hard to break the bonds. I don't come from a religious household (except my grandmother, may she rest in peace), but during my entire childhood, I don't remember anyone talking about God as if there is any doubt as to His existence. "Only God knows", "God bless you", "Pray to God".... The entity 'God' exists. Only later the quesion was raised as to whether to believe in Him. The question of His existence was never raised. You can believe in Him or not believe in Him. In either case, He exists.
Now I'm not completely sure people who lived in a society where God wasn't taken for granted will understand exactly what I'm talking about. I think that most of you, though, DID grow up (or are still growing up) in such a society. I hope what I said makes sense, as it's kind of hard to get across on paper (or whatever media) what I mean.
The reason it seems so obvious to me people believe because they learn to believe is that so few people believe in something that is radically different from what they were taught. There are extremely few conversions between religions. Statistically, it is negigible. Isn't it strange that almost all people believe what people who are close to them believe? There are very few people who believe in one thing when their parents, teachers and friends all believe in another. It is quite obvious to me that belief comes first from without and not within.
I wish to explicate a small point: the cat story is not exactly about conditioning. It is more about hardwiring. I just thought I should mention that to prevent confusion. The cat's physical brain was changed, it was not taught anything. The reason we believe in God is because of conditioning, not a permanent messing with our nervous system.* But the cat story shows the full potency of our brain. The cat cannot see the vertical lines, yet they exist. It was "told" in its youth all that exists are horizontal lines. People are told in their youth God exists.
* I am making a distinction here between physiological and psychological. I do not want to get into a discussion as to whether one exists, at least not here.