The Decline of the Druids
From about the first century, under heavy Roman influence, the institution of druidry began to fade away, and with the establishment of European Christianity, it disappeared almost entirely. The influence of religion, however, was not responsible for this decline, which instead was largely an issue of language, law, and, ironically, religious tolerance.
What contributed the most to the druids' disappearance was language. Romans were generally religiously neutral. They were not terribly interested in eradicating Celtic religion, except where druid power influenced uprisings and disturbed their carefully planned social order. Further, the Romans identified the Celtic gods as simply foreign versions of their own deities. They encouraged syncretism between the pantheons, which of course benefited the Roman rulers more than the druids.
The Celts under Roman rule were Roman citizens, and they were encouraged (sometimes by force) to make use of the Roman legal system, whose operative language was, of course, Latin. Latin eventually supplanted the Gallic languages, which the druid oral tradition relied on. Druid schools crumbled, and the hierarchical structure of druidry failed. What remained of druidry was pushed into Ireland and Scotland, outside the reaches of the Roman Empire.
The Druids and the Christians
With the coming of the Christians, the use of Latin became a matter of religion as well. The old languages were relegated to the pagans and derided as the tongue of uneducated peasants, and so whatever remained of the druidic intellectual structure declined rapidly.
There are still reports of druids operating all the way into the twelfth century. But because the ancient institutional structures were gone, the initiatory teachings of druidry passed from one individual to the next. Eventually, druids were viewed as little more than solitary healers, wizards, or even common fortune tellers.
The Christians in Ireland were of a different sort than those who had earlier conquered other parts of Europe. They were primarily scholarly monks. Although they were largely disdainful of the pagan religions, they retained enough respect for the institution of druidry that many elements of Celtic theology became intertwined with Celtic Christianity.
In their histories, the Christian monks painted an often contradictory view of druidry. Druids portrayed in the monks' accounts of Ireland's ancient heroic past are stern, powerful, and dignified. Many are even portrayed as wise prophets, foretelling the coming of Christianity to the island. Later, reflecting real-life political power struggles, druids were depicted more as devious wizards or scheming, superstitious pagans. This strange dichotomy persisted into medieval times — Irish ancestral pride perhaps at odds with proper Christian sentiment.
Not all of the recorded predictions of the coming of Christianity were positive. The druids of the court of Loeghaire, Lochru, and Lucat-Mael made a dark prediction that a “shaven-headed foreigner” would appear and seduce the people and that this stranger's new doctrine would forever supplant the religion of the druids. This stranger of course was St. Patrick. Patrick is credited with the conversion of Ireland to Christianity.
Not everything was peaceful, however. The conversion of the Celts, whose spirituality already gave them a unique understanding of Christian ideas of trinities, baptisms, prophets, and resurrected gods, were not too resistant to the new faith. Many of Ireland's druids, on the other hand, faced a loss of power, prestige, and property under the new regime, and conflicts between local druids and Rome's bishops were inevitable. This was not helped any by the animosity that St. Patrick exhibited toward the druids.