In Caesar’s Gallic Wars, are found 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.