Paganism

Long before recorded history, humans lived (out of necessity) much closer to the natural cycles. It is out of this affinity with nature that humans first began to personify the inherent energies of the land as individual Gods and Goddesses. In this manner, the ancients attempted to make sense of what must often have been a very illogical universe. The earliest peoples would have practiced a lifestyle very closely tied to the cycles of hunting and gathering (for obvious reasons, anthropologists refer to them as "hunter-gatherers").
Some research supports the theory that the most ancient formalized religion was one of ancestor worship. During those periods just after the beginnings of recorded history, we see evidence, which points towards this conclusion. The earliest evidence of pre-Christian Roman culture indicates that the Romans practiced some forms of ancestor worship. Early Roman culture borrowed extensively from the Etruscans; in fact many scholars accept that the Etruscan culture had an enormous impact on the civilizations of their time. Etruscan pottery has been found in excavations throughout the "Roman Empire", even as distant as Britain.
In addition to the reverence placed on the ancestors of a tribe or clan, the role of women appears to have remained significant. Long before the advent of patriarchal societies, an individual's family ties were determined by the line of descent from the mother's side. Some cultures still determine descent in this manner, however they are now greatly in the minority. The patriarchal lineage appears to have its strongest foothold in warrior societies or their offshoots. As the Celts had strong warrior women, it is decidedly unclear if patriarchy is the cause or result of such societies.
Eventually, the hunter-gatherer societies changed. Their pastoral culture slowly transformed either by natural evolution of by force, into an agrarian one. This culture was delineated less by the cycles of the hunat, and more by the ability to sow and reap the corps of the field. Several clans may have settled in a single area, and by developing more specialized functions became a small village. Gods and Goddesses of the forest and the wild places slowly began to be passed by and occasionally forgotten: their places userped by those of field or cattle and flocks. As these areas grew from village to towns and larger urban areas, so the Gods and Goddesses changed. "New" deities appear on the scene and new aspects of the Older Gods emerge: smithcraft, milling, and the like needed patrons to oversee them. In the East, and in other lands, many bands still clung to nomadic ways. Others still, spent some or all of their energies on the process of war. The divine spirits of these warrior bands appear to be almost exclusively male. Within those societies, it became the physiological strength of the male, which is of prime importance. As these bands grew and conquered others, so their patriarchal views of the order of things began to spread.
History is written by the conquerors, and re-written by the next one. So too, is religion. As the patriarchal bands, and later, patriarchal cultures, conquered and expanded their territories, so they re-wrote the histories and practices of the peoples they conquered. Several fine examples of this have much later dates in history:

Sometimes, a culture or people will resist. This resistance may be covert or overt in nature. Many traditional Celtic ways were once forbidden, such as bagpipes and the Celtic languages themselves. In more modern times, these pieces of culture are appreciated and treasured. Truly, one small battle won.
Historically, the Celts themselves were conquerors of the lands we now see as "Celtic"> In truth, there is not a surviving culture that wasn't at least partly warlike in its history. The Celts that we are so familiar with, those of the islands of Britain, are said by some to be descended from a cultural entity known as the Indo-Europeans. These people may have originated in the areas of the Northern Indian sub-continent. Their migrations north and westward contributed to the cultures we are now familiar with in India, the Iberian Peninsula, and the British Isles. It is probable that the contributions to these cultures were actually userptions by way of conquering other tribes along the way, however there is little hard evidence to prove any single theory. What we do have are many cultural similarities ranging from the Brahmin of India to the Druids of Northern Europe; from the intricate Knotwork patterns of the insular Celts to the key designs of the Greeks and the Minoans.

The History of Paganism

(Beware: this is the long version, beginning not at Gardner but at the ancient times.)

Paganism is actually a very ancient religion; however, since so few historical texts have been found intact and few can be translated, modern Pagans have been forced to essentially reconstruct the ancient religions to the best of their ability.
Some of the oldest examples of Paganism are the statues of motherly figures, with ample figures, that demonstrate the Mother aspect that would have been primarily important to the early people. These statues, from a very early time period (35,000-10, 000, b.c) have been found all over Europe. The Venus of Willendorf, or Willendorf Goddess, is a commonly used example.
Palaeolithic cave art seems to suggest that the early peoples painted the animals in a way connected with hunting--perhaps to draw the animals' spirits to the hunters, to ensure a good hunt. This is likely a form of sympathetic magic.
About 80,000 years ago, still in the Palaeolithic period, burials appeared--burials, the mark of the human species. While burials do not neccessarily mean a religion, thy do indicate a belief in the afterlife. One sixth of known burials of the Palaeolithic times were sprinkled with red ochre, and ornaments have been found in conjunction with the burials.
Moving on to the Neolithic time period, we find the henges: archaelological sites that seem to have more of a spiritual purpose than functional. Henges seem to be a phenomonon confined to Britain, and does not appear to have reached the main land. One of the most famous of the henges is Stonehenge, which stand in the middle of Salisbury Plain, in the chalk upland of Wiltshire. Stonehenge was actually built over a long time period, not just the period in which it originated (the Middle Neolithic). Henges are not necessarily the same as stone circles. Henges, as is said above, are those sites which appear to be more spiritual than functional.
Celtic trads are probably some of the most common today; it is nearly impossible not to come into contact with at least one person or group with Celtic associations when you are a pagan. However, the Celts are one of those peoples who left few actual records behind them, and much of the current traditions have had to build onto the somewhat scanty knowledge concerning the Celts.
The Celts had many gods/goddesses; there are records of over 400 deities, at least 300 of whom are only mentioned once. Thus, it is hard to tell if the Celts regarded the deities as seperate, or as aspects of one whole. Many of the deities are very much localized, distinct to one region or another. They had a strong belief in the afterlife, at least; archaeological information supports that fact.
One of the divisions of the Celts was the Druids, who lived in communities that seem to have been mostly male and intellectual. There is little available information on the Druids, regrettably; and some of what is available is from unreliable sources, the Druids' enemies.
In the category of classical religions fall the Romans and the Greeks. These peoples believed that one must pay homage to all the gods, not merely the ones whose attributes you most wanted, because if you neglected a god or goddess, you would miss out on that aspect of life they represented.
As the Romans did not have an insular religion when they conquered the Greeks, they essentially took over the Greek pantheon. The only Roman god who does not have a Greek counterpart is Janus, a creation god. The Romans did have several myths and legends that were their own, but unlike the Greeks (who mainly used myths to provide an example for the ideal of human behavior) the Romans preffered myths that glorified heroism, bravery, and duty. Both pantheons were very polytheistic (believing in more than one deity) in nature.
The Egyptians were extremely polytheistic; they had hundreds or thousands of gods and goddesses. The main Egyptian deities were Isis and Osiris, along with Horus. Isis was the creation goddess; she was the wife of Osiris, who was tricked and killed by his evil brother Set/Seth. Isis brought Osiris back to life long enough to conceive their son, Horus, falcon-headed god of the sun. After Osisris died, he became the God of the Dead and is commonly pictured as a mummy.
Another important set of goddesses were the four sisters: Isis (who was probably the most important Egyptian deity), Nephthys, Selket (goddess of scorpians), and Neith (goddess of night). These are the four sisters who are depicted in Tutankamen's tomb, guarding the four corners of his sarcauphagus (outer coffin).
In the Middle Ages, Christianity reigned and Paganism went into hiding. However, it was revived in the 1800s and 1900s, becoming what is Paganism today. Gerald Gardner is often regarded as the father of modern Paganism; however, there is an argument that Aliester Crowley was the father of modern Paganism. While there is little doubt that Crowley did have a major impact on modern Paganism, I tend to suscribe to the theory that Gardner was in fact the founder of Wicca.
When the last of the anti-Witchcraft laws were repealed around the 1940s, Gardner published books that were openely about Paganism, thus beginning the revival of Paganism in England. It spread to the states, notably aided by Buckland and his wife, Rosemary, who had been initiated by Gardner. Since then, Paganism has continued to be one of the fastest-growing religions in the United States, and many other places--Australia, for example--have a high Pagan content.

History of Paganism 101: The Roots of Nature Spirituality

The History of Paganism