I wonder if there are any resources that tell the story of paganism, from as far back as we have artifacts/resources from, on the pagan religions. Everything I find is from a religious site - whether a pagan site who currently worships said deities, or from a christian site talking crap about them. I'm interested in what atheists have to say about the history, how it started, how did such belief systems develop? With the Celts I can only get back as far as the Druids, but I'm not sure where they got their ideas from.
I'm just interested in how these concepts emerge in society, like what happened in their societies to make them think it was deities? What were the probable natural causes that they they were trying to explain and how did they make the jump to thinking a god or goddess was responsible for such things?
Gerald Gardner is considered by many to be the father of modern paganism, more specifically that of wicca.
As a former pagan, I found that many in the pagan community pulled bits and pieces from many if not all the pre-christian religions. Everything from the Celtic wheel of the year to the Jewish mystical tree of life to the chakras and Karma. It's all interwoven into a mesh or patchwork of ritual, arcane "knowledge" and belief that they are doing something more correctly than others.
It's true that it's very hard to find paganism-related things that aren't either from the perspective of pagans who are glowing about it, or Christians demonizing it. That's why it was very hard for me to find other atheist ex-pagans too.
They came up with religions in the same way that others came up with religions I guess...and I believe that one's environment has a lot of influence on religious views.
To add to what Bryan is saying, most pagans built up a myth about being part of an old, ancient wisdom. I met a few who said that annoys them, because neo-paganism is really a new religion. Not only because it's a mix-and-match of many different traditions but also because it's probably been greatly gentrified compared to the pre-bronze-age traditions.
I guess what I'm trying to find is not about paganism today, but about the ancient pagan religions themselves – maybe what I meant to say was the ancient polytheistic religions. For instance, we know the Romans stole from the Greeks so Venus is really Aphrodite, but where did she come from? Where did Rowan come from? If you keep going back from culture to culture, I wonder how far we can go back to the first cultures to create these ideas. How did these goddess and gods get invented to start with?
A great book is Cows, Pigs, Wars and Witches by Marvin Harris. Basically, the consensus seems to be that people invented god/desses to explain what they did not understand, which, to a cave man, was quite a lot! People still use god this way, as a "god of the gaps". If they are missing information they just insert "gods did it" instead of either admitting they don't know or trying to find an alternate explanation. I imagine ancient man developing gods and religion out of a fear of the world and the unknown. So, yes, the first cultures of the world had many magical and religious beliefs but I don't think you can trace them as being directly related to the beliefs of today. Many beliefs die out and new ones take their place. It is part of human nature to assume other things have a consciousness like ours (the sun, the moon, animals, the weather) but that doesn't mean it's true or necessary :)
I was Greek Re-constructionist and one of the things I sort of hand-waved when researching was the fact that most of the educated people didn't believe in religion. The city temple offices were patronage or nepotic plum assignments. In fact, because these people tended to do most of the writing, there's little evidence that anyone actually sincerely believed, though it is assumed that the uneducated did. It really seems a case of "plus ca change plus c'est la meme chose". So if the Greek religion, at the height of Greek civilization, was aknowledged by the elite to be a fantasy or a tool to control the unwashed masses...it's difficult to say where it all started.
But I do imagine the tales, like all mythology, began as a way to explain things. Lightning as a weapon of the gods is an old and widespread myth.