Maximus of Tyre testified to the worship of oak trees by the Druids, and concluded that since these are the same trees of the oracle at Dodona, the Celts worship Zeus and liken him to an oak tree. Pliny the Elder also testified to the worship of the oak trees, and believed that the druids were actually named after the oak, since the word for oak in Greek is drus (δρυς), and it turns out also in the Celtic languages (derw, dair).
Oak trees were usually sacred to thunder gods, Hector Munro Chadwick wrote about this in his study “The Oak and the Thunder God” in 1900. We see how St. Boniface uprooted the oak of Donar, the parallel of Thor, identified in Latin hagiography as “the tree of Jove” (i.e Jupiter). When the Saxons saw that nothing was happening to Boniface, they immediately all converted to Christianity. The Baltic thunder god Perkūnas is also closely related to the oak. And of course, in Hebrew there is also the connection between the god “El” and the tree of “Ela” or “Alon”.
You would say that the similarity to the worship of El is a nice coincidence, but considering that in Roman times there was a practice of worshiping “columns of Jupiter”, and such columns were found in a variety of Roman-Celtic sites such as Mainz, Maastricht and even Budapest – these columns actually symbolized the tree of the Axis Mundi, and were often have scales and bumps that resemble those on the surface of the oak trunk, which protect it from pests. These columns are the counterparts of the Hercules columns, or Melqart columns, which were placed in the center of Phoenician temples. The Celts met the Phoenicians in Spain or Ireland, and may have adopted this worship over time. One of the theories regarding the distribution of the Celts claims that all the Celtic languages and all the peoples called Celtic are not necessarily genetically related peoples, but rather an elite that settled in trading stations on the shores of the Atlantic Ocean and the Celtic language was their lingua franca. Some of them may even be of Semitic origin, or at least those who founded the colonies. In any case, these pillars and the worship of the oak trees may be of the same origin.
Strengthening this claim that the Celts adopted local rituals may come from Arthur John Evans’ study of Stonehenge from 1889. Evans points out that Stonehenge was actually a place of worship around which the circles and stones were built, and this place may have been a sacred oak tree, symbolizing the same god as the father of all. Evans shows parallels in Greco-Roman art depicting sacred trees associated with stone pillars or trilithic structures (i.e megaliths built of 3 stones), and thus concludes that these monuments are generally associated with sacred trees. The remains of the sacrifices found in the area also strengthen his claim that the Celts adopted this form of worship that matched their cosmology. This is of course if we adopt the approach that the Celts were late invaders who preserved a cult that had existed in the place since time immemorial.
The god of thunder, the father of the Celtic gods is called Taranis, he is symbolized as a naked god carrying a wheel and lightning. The wheel symbolized his chariot of the sun crossing the sky. The wheel was a recurring motif in Celtic art, and was usually used as a protective and luck amulet among warriors. The warriors, by the way, fought like Taranis, naked, and painted themselves blue like the color of the stones in the pits of Stonehenge. Julius Caesar describes this blue color as the color of glass, and it is not clear why. One of the explanations is because when lightning strikes the sand, glass is sometimes formed in the form of thin tubes called Fulgurite, and their color is sometimes blue, which is of course related to a multitude of variables such as the type of sand, the intensity of the lightning, etc., but when you are a pre-scientific pagan, nothing is clear to you.
And more about the worship of the oak trees and the god of thunder: among the tools discovered in the Hallstatt culture were found decorations and ornaments of birds. Since this god is the god of the sky, whoever is able to fly and dwell in the sky is also holy, and we see many birds that are considered sacred in Celtic culture. The Celts used to practice divination based on the flight of birds, some also claim that traditional Celtic music tried to imitate birdsong. Birds were considered messengers connecting the sky and the earth, and represented the souls on their journey to the other worlds, or rather, the ability to fly symbolized for them the soul leaving the body and embarking on a journey.
The bird sacred to the druids was the wren, whose name also happens to be very similar to that of the druids Drwy, and it also usually lives among the branches of the oak tree. Harming this bird or its nest is considered a taboo, transgressing it brings bad luck. However, on one day a year it was allowed and even desirable to hunt these birds, on the day of the winter solstice, perhaps to hasten the return of the sun symbolized by the god. In fact to this day in the Isle of Man and Ireland there is a custom at Christmas to hunt these birds, which is called Wren Day. Celtic legends usually praise the skill of the warriors as a measure of how accurate their stone throw was, so much so that they were able to snipe birds.
Entheogens and sacred herbs 