The Druids were the learned class of ancient Celtic society — philosophers, priests, judges, historians, poets and healers. Julius Caesar, one of our main sources, reported that Druidic training took up to twenty years and was conducted entirely through oral transmission. The Druids held that the soul was immortal and transmigrated between lives — a belief that the Romans found philosophically sophisticated enough to compare with Pythagorean doctrine.
The three grades of the Druidic order were the Bards (preservers and transmitters of cultural knowledge through song and story), the Ovates (specialists in divination, healing and the communication between worlds) and the Druids proper (philosophers, judges and ritual specialists). Modern Druidry has revived this three-grade structure as a framework for spiritual development.
The sacred groves (nemeton) were the Druids' primary sacred spaces — clearings in forest where the boundary between the human world and the Otherworld was thin. The Romans destroyed every nemeton they found, understanding that destroying the sacred space destroyed the spiritual centre of Celtic resistance. The last recorded nemeton was destroyed on the island of Anglesey (Ynys Môn) in 61 CE.