In the Celtic religion, the modern words Druidry or Druidism denote the practices of the ancient druids, the priestly class in ancient Celtic societies through much of Western Europe north of the Alps and in the British Isles. Druidic practices were part of the culture of all the tribal peoples called Keltoi and Galatai by Greeks and Celtae and Galli by Romans, which evolved into modern English "Celtic" and "Gaulish".

Modern attempts at reconstructing practising druidism are called Neo-druidism.

From what little we know of late druidic practice it appears deeply traditional, and conservative in the sense that the druids were conserving repositories of culture and lore. It is impossible now to judge whether this continuity had deep historical roots and originated in the social transformations of late La Tene time, or whether there had been a discontinuity and a druidic religious innovation. The etymological origins of the word druid are varied and doubtful enough that the word may be pre-Indo-European. The most widespread view is that "druid" derives from the Celtic word for an oak tree (doire in Irish Gaelic), a word whose root also meant "wisdom."

From what little we know of late druidic practice it appears deeply traditional, and conservative in the sense that the druids were conserving repositories of culture and lore. It is impossible now to judge whether this continuity had deep historical roots and originated in the social transformations of late La Tene time, or whether there had been a discontinuity and a druidic religious innovation. The etymological origins of the word druid are varied and doubtful enough that the word may be pre-Indo-European. The most widespread view is that "druid" derives from the Celtic word for an oak tree (doire in Irish Gaelic), a word whose root also meant "wisdom."

Their influence was as much social as religious. Druids used not only to take the part that modern priests would, but were often the philosophers, scientists, lore-masters, teachers, judges and councillors to the kings. The Druids linked the Celtic peoples with their numerous gods, the lunar calendar and the sacred natural order. With the arrival of Christianity in each area, all these roles were assumed by the bishop and the abbot, who were never the same individual, however, and might find themselves in direct competition.

Our historical knowledge of the druids is very limited. Druidic lore consisted of a large number of verses learned by heart, and we are told that sometimes twenty years were required to complete the course of study. There may have been a Druidic teaching center on Anglesey (Ynys Mon) centered on magical lakes, but what was taught, whether poetry, astronomy or whether possibly even the Greek language, is conjecture. Of their oral literature of sacred songs, formulas for prayers and incantations, rules of divination and magic, not one verse has survived, even in translation, nor is there even a legend that we can call purely druidic, without a Christian overlay or interpretation.

Much traditional rural religious practice can still be discerned beneath Christian interpretation, nevertheless, and survives in practices like Halloween observances, corn dollies and other harvest rituals, the myths of Puck, woodwoses, "lucky" and "unlucky" plants and animals and the like. Orally-transmitted material may have exaggerated deep origins in antiquity, however, and is constantly subject to influence from surrounding culture.

Roman Sources

We find in Caesar's Gallic Wars the first and fullest account of the Druids. Caesar notes that all men of any rank and dignity in Gaul were included either among the Druids or among the nobles, two separate classes.

The Druids constituted the learned priestly class, and they were guardians of the unwritten ancient customary law and had the power of executing judgment, of which excommunication from society was the most dreaded. Druids were not a hereditary caste, though they enjoyed exemption from service in the field as well as from payment of taxes. The course of training to which a novice had to submit was protracted. All instruction was communicated orally, but for ordinary purposes, Caesar reports, the Gauls had a written language in which they used the Greek characters.

No druidic documents have survived. "The principal point of their doctrine", says Caesar, "is that the soul does not die and that after death it passes from one body into another". This led several ancient writers to the unlikely conclusion that the druids must have been influenced by the teachings of the Greek philosopher Pythagoras. Caesar also notes the druidic sense of the guardian spirit of the tribe, whom he translated as Dispater, with a general sense of Father Hades.Writers like Diodorus and Strabo with less firsthand experience than Caesar, were of the opinion that this class included Druids, bards and soothsayers.

Pomponius Mela is the first author who says that their instruction was secret and carried on in caves and forests. We know that certain groves within forests were sacred because Romans and Christians alike cut them down and burned the wood. Human sacrifice is sometimes attributed to Druidism; it was an old inheritance in Europe, (although this might be Roman propaganda). The Gauls were accustomed to offer human sacrifices, usually criminals.

Britain was a headquarters of Druidism, but once every year a general assembly of the order was held within the territories of the Carnutes in Gaul.

Cicero remarks on the existence among the Gauls of augurs or soothsayers, known by the name of Druids; he had made the acquaintance of one Divitiacus, an Aeduan. Diodorus informs us that a sacrifice acceptable to the gods must be attended by a Druid, for they are the intermediaries. Before a battle they often throw themselves between two armies to bring about peace.

Druids were seen as essentially non-Roman: a prescript of Augustus forbade Roman citizens to practise druidical rites. In Strabo we find the Druids still acting as arbiters in public and private matters, but they no longer deal with cases of murder. Under Tiberius the Druids were suppressed by a decree of the Senate, but this had to be renewed by Claudius in 54 CE.

In Pliny their activity is limited to the practice of medicine and sorcery. According to him, the Druids held the mistletoe in the highest veneration. Groves of oak were their chosen retreat. When thus found, the mistletoe was cut with a golden knife by a priest clad in a white robe, two white bulls being sacrificed on the spot.

Tacitus, in describing the attack made on the island of Mona (Anglesey or Ynys Mon in Welsh) by the Romans under Suetonius Paulinus, represents the legionaries as being awestruck on landing by the appearance of a band of Druids, who, with hands uplifted towards heaven, poured forth terrible imprecations on the heads of the invaders. The courage of the Romans, however, soon overcame such fears; the Britons were put to flight; and the sacred groves of Mona were cut down.After the 1st century CE, the continental Druids disappeared entirely, and were only referred to on very rare occasions. Ausonius, for instance, apostrophizes the rhetorician Attius Patera as sprung from a race of Druids.