There is no one simple ancient source of knowledge on the Druids. What knowledge there is can only be gleamed from several ancient texts, some of which were anti-Celt anyway. Caesar says that the Druids believe that their religion forbids them to commit their teachings to writing …. Hence the difficulty of saying with any accuracy what the Druids believed and how they functioned in Celtic society.
Again according to Caesar, who took a great interest in them, it took some 20 years of study to become a Druid. Men and women could be Druids, as could rulers. Women indeed had great equality in Celtic society, and could be rulers, warriors as well as Druids. Boudicca is certainly the best know example of a warrior-queen, but Tacitus says in his Annals it is not the first time that Britons have been led into battle by a woman.
Anglesey or Mona as it was called was held to be the very centre of Druidism. It is speculated that this is why Suetonius Paullinus took such trouble to invade Mona and destroy the Druids there, who were seen to be the source of rebellion against the authority of Rome.
The famous bard Taliesin refers to its place in Welsh history in the verse, There will come men to Mona to be initiated into the ways of the wizards, (quoted by Jean Markale in The Celts).
There is speculation about the beliefs of the Druids, but it is certain that they believed in reincarnation, and there is strong evidence for their belief also in an after-life before reincarnation took place. They were also recognised by the Greeks and Romans as having an advanced knowledge of astronomy and writers such as Pliny praised their knowledge of medicine.
The Coligny calendar is a revealing relic which also shows the Druidic knowledge of astronomy, and confirms Caesar's assertion that the Celts used a lunar calendar and counted their time not in days but in nights.
Another remarkable discovery is that of the Gundestrup Cauldron, a silver dish capable of holding 130 litres, which historians closely associate with the Druids. It confirms the importance of Cauldrons in Celtic society, and provided the idea behind Rhiannon's search for the Cauldron of Wisdom in the book. On the Gundestrup Cauldron can be seen figures such as the horned God Cernunnos, sitting in a lotus position, as well as a dead warrior being held over a cauldron, whilst living warriors walk away from it.
This quote from the Greek geographer Strabo describes the Druids:
Amongst all the Gallic peoples, generally speaking, there are three categories of men who are held in exceptional honour: the Bards, the Vates, and the Druids. The Bards are singers and poets; the Vates Diviners and natural philosophers; whilst the Druids, in addition to natural philosophy, study also moral philosophy. The Druids are considered the most just of men, and on this account they are entrusted with the decision, not only of the private disputes, but of the public disputes as well; so that in former times they often arbitrated cases of war, and made their opponents stop when they were about to line for battle. …However, not only the Druids but others as well, say that men's souls, and also the universe are indestructible, although both fire and water will at some time or other prevail over them.
(Strabo, Geographica, IV, 4, c 197.4)
Quote taken from Druids, Preachers of Immortality by Anne Ross