Although the origin of Druid religion is still being discussed, it is certain that this is only adhered to by the Celts. Julius Caesar wrote, for example, that the Gallic Druids were educated in Britain. He also suspected that the origin of Druidism can be found there. Other historians suspect it originated near the Black Sea. I’ve never been able to find any evidence of either claim. However, it is clear that it stretched from Ireland to Turkey.
During the days of the Great Roman Empire, Druidism is increasingly and often suppressed. The subsequent rise of the Christian faith is almost the final blow to the Celts and Druids. As Europe slowly moves towards the Middle Ages, part of Druidism is ent into the Christian faith.
It was by what later turned out to be the wrong conclusion of scientists and writers in the 18th century that megalithic structures such as Stonehenge would have been built by druids. That there has been a resurgence in Britain. As we now know, these structures can be traced back to about 2 thousand years earlier. From this period also comes the image that the Druids would be old men with long gray beards. Fortunately, the misconceptions of that resurgence have since faded away from contemporary Druidism. And is therefore Druidism now accessible to women, children and men. And there is no longer any need to have a Celtic background.
Although the Celts and druids chose to capture as little as possible, or at least little more of this can be found. Can we still distill a lot of information from the writings of their Greek and Roman contemporaries. The manuscripts of medieval British and Irish monks still tell stories of Celtic legends. As well as the archaeological finds that have been made there and are still being excavated. The Ogham can also provide us with some information.