I recently read a very interesting theory about the role of Druids in Celtic society. Sean B. Dunham in an article Caesar's perception of Gallic social structures puts forth the theory that we may have the role of the Druid all wrong. Whereas the modern view of them is that of priests, that in actuality they had the tripartite duties of priests, philosophers and judges. The ancient world didn't have the separation of church and state that we do now. Instead of being a mysterious magician priests, they actually were an oligarchical elite with both judicial and religious duties.

If so, it appears that Caesar had a much clearer picture of the Druids and the Celts than many have given him credit for. He understood the importance of the Druids which quite possibly accounts for his very harsh treatment of them. They held the history, the law, the wisdom, the religion of their society. Once this was removed, much of what made them Celts also eventually vanished.

At least this is my impression from what I've read and studied from historical sources. The problem with Druids and Celts is that since the late 17th century, their history has undergone several revisions, and what now passes for ancient Celtic religion....is actually a 20th century century. So much of what they really were is now lost in hazes of rewrites and fantasy.