There had once been a great Hyperborean culture (beyond the north wind), from whom the Athenians had learnt their religion. Even by the time of Jesus, the Druids still exerted power over people’s faith. The Irish story of Lugh (the Sun’s solar cycle), the druid Trinity was adapted and adopted into Christianity.
The Quran mentions people who worshipped in groves, and a prophet Idris who is mentioned in Welsh traditions. There has been a reappraisal of Pre Roman Northern Europe. It was far more civilised and ordered than thought before. Classical scholars wanted to validate Christianity, Roman post colonialism. It appears that northern Europeans had probed ancient China at least as far back as 2000 BC. Artefacts found in graves show European Skeletons dressed in Tartan and tall hats likened to the Welsh hats. They were probably priests. There was also a people called the Tocharians, a white blond people who brought Buddhism into China. The druids seemed interested in the rising and setting places of the sun and invented the calendar. They may have reached Japan.
This was not a Celtic fringe religion, but has been found across northern and central Europe and the steppe. Many of Alexander’s mercenaries claimed to be Celts and had some kind of Druidic beliefs.