There are two main problems when looking at the evidence for the Celtic priesthood known as the Druids:
What does seen clear however is that the Druids belonged to a class that included high ranking holy men whose main function was to oversee the religious life of the community. Pomponius Mela called them the "professors of wisdom". Caesar also relates their judicial powers in matters of murder cases and other areas which affected "either individuals or the public".
Information relating to Druid ceremonies relates largely to sacrifice. Strabo tells us how victims were stabbed in the back and divinationís made from their death throes, he also recounts other forms of human sacrifice including shooting to death by arrows or impaling and the burning of victims inside a huge wicker man. Caesar also writes of the wicker man whose background and meaning is still not explained. Tacitus refers to British human sacrifice on the Isle of Anglesey where, in a sacred grove, altars were drenched with human blood and entrails and consulted by the Druids for divinitory purposes.
The only detailed account of a Druid ceremony is given by Pliny who describes the cutting of mistletoe from an oak tree, a very rare occurrence. The rite was held on the sixth day of the moon and preparations were made for a feast and a sacrifice of two white bulls. A white robed Druid climbed the tree and cut the mistletoe with a golden sickle, which was caught as it fell on a white cloak. The bulls were then sacrificed.
Classical writers also provide us with some insights into the belief system of the Druids. Pomponius Mela tells us that the Druids believed "that souls are eternal and there is another life in the infernal regions", an item of Druidic belief echoed by other classical writers. As a class the Druids were also the repositories of much practical lore including knowledge of astronomy and calendrical computations. Caesar attributes the Druids with "much knowledge of the stars and their motion, of the size and shape of the world, the movements of the heavens and of the stars".
The Druids were also the chief providers magic and supernatural wisdom in Celtic society. Ritual cursing "riddles and dark sayings" (Diogenes Laertius) are ascribed to them. It is also likely that they had some knowledge of herbs and herb lore.
It seems likely that the British Isles was the centre of Druid beliefs and Caesar tells us that Gaulish Druids were sent to Britain to be trained. It is probable that British Druids met together regularly each year on a fixed date in the same manner as their Gaulish counterparts, who assembled at a sacred place in the land of Carnutes (ancient Chartres).
The Irish literary sources provide us with many references to Druids. In the Book of Invasions Partholon, one of the invaders, arrives with three Druids, Finn (hero of the Fionn Cycle) is reared by a Druidess and the goddess Bidgit is born in a Druids household. The most famous literary Druid is however Cathbadh, a member of the Ulster King Conchobarsí household and a figure of great importance. Cathbadh is well recorded for his great powers of divination and seeing into the future.
The Druidic religion came to an end as a result of successive rounds of persecution by the Roman authorities. In AD 64 the Romans sacked the island of Anglesey in an effort to bring Druidism to a final end. There are a number of possible reasons why the Romans pursued this course of action including the wish to stop the human sacrifices (which they found disgusting to their sensibilities) and the wish to put an end to what might have been a nationalistic and political focus inherent in Druidism.