Druids were highly respected for the learning and wisdom. Caesar said that they studied "the stars and their movements, the size of the cosmos and the earth, the nature of the world, and the powers of immortal deities." (Despite this sophistication that mirrored the Roman world, Caesar also described the frequency of human sacrifice, likely in order to paint them as inferior to the civilized Romans, and therefore worthy of conquest.)
Other classical writers, Diodorus Siculus and Strabo, claimed the druids were so revered that they could stop an impending battle between armies simply by intervening. They were philosophers and knowledgeable about religion. Another Roman, Pomponius Mela, writing around the same time as Julius Caesar, made the first remarks about how their learning was conducted in secret, taking place in caves or hidden in forests.
Caesar claimed it could take 20 years for a druid to properly learn the required lore, because nothing was committed to writing, but needed to be thoroughly memorized. Yes, there was a written language in Gaul, but there exists today no single written line of "druidic lore" that can be verified as authentic. The forgeries of Edward Williams/Iolo Morganwg do not count.
In the centuries after the conquest of Gaul, the Romans took measures to wipe out the druids. Pliny the Elder wrote that Emperor Tiberius (14-37CE) banned druidic practices (and soothsayers) in order to stop human sacrifice.
Druids seem to have survived in the British Isles a little longer, until Christianization spread. Their role as carriers of oral tradition and law did not disappear, however: it survived with a different name for the purveyors. That name was "bard." Tomorrow we'll look at one of the early and most famous bards, of whom some say it can be argued that he was one of the last druids: Taliesin. See you then.