Although since Christian times Druids have been identified as wizards and soothsayers, in pre-Christian Celtic society they formed an intellectual class comprising philosophers, judges, educators, historians, doctors, seers, astronomers, and astrologers. The earliest surviving Classical references to Druids date to the 2nd century B.C.E.

The oak (together with the rowan and hazel) was an important sacred tree to the Druids. In the Celtic social system, Druid was a title given to learned men and women possessing "oak knowledge" (or "oak wisdom").

Some scholars have argued that Druids originally belonged to a pre-Celtic ('non-Aryan') population in Britain and Ireland (from where they spread to Gaul), noting that there is no trace of Druidism among Celts elsewhere - in Cisalpine Italy, Spain, or Galatia (modern Turkey). Others, however, believe that Druids were an indigenous Celtic intelligentsia to be found among all Celtic peoples, but were known by other names.

The image shown here is a representation of the Celtic Tree of Life. The Tree was a central part of early Celtic spirituality. To the Celts, the tree was a source of sustenance, a bearer of food, a provider of shelter and fuel for cooking and warmth. Trees were also associated in the Shamanic beliefs of the Druids and other Celtic peoples with the supernatural world. Trees were a connection to the world of the spirits and the ancestors, living entities, and doorways into other worlds.

Wood from sacred trees had magical properties, which was reflected in the Celtic Ogham alphabet, wherein each letter represents a particular sacred tree (modern Ogham divination is based on the uses and importance of these sacred trees to the Celtic people). Some trees provided food, some wood for making hunting weapons; others were sacred to the fairy-folk or to the Gods. In Celtic creation stories, trees were the ancestors of mankind, elder beings of wisdom who provided the alphabet, the calendar, and entrance to the realms of the Gods.
 

The most sacred tree of all was the Oak tree, which represented the axis mundi, the centre of the universe. The oak was the doorway to the Otherworld. Its Celtic name, daur, is the origin of the word door. The word Druid, the name of the Celtic Priestly class, is compounded from the words for oak and wise- a Druid was one who was "Oak Wise," meaning learned in Tree magic.

Countless Irish legends revolve around trees. One could fall asleep next to a particular tree and awake in the fairy realm. In Celtic legends of the Gods, trees guard sacred wells and provide healing, shelter, and wisdom. Trees carried messages to the other realm, and conferred blessings- to this day, trees can be seen in the Irish countryside festooned with ribbons and pleas for favors, love, healing, and prosperity.

The interlaced figures known popularly as Celtic knots represent sacred trees and plants, and the sacred animals of the forest. The Green Man or foliate god is the animus of nature; the spirit of the forest and of the hunt, and is pictured as a spirit face in the form of gathered leaves and sprouting tendrils

The Solar cross is probably the oldest religious symbol in the world, appearing in Asian, American, European, and Indian religious art from the dawn of history. Composed of a equal armed cross within a circle, it represents the solar calendar- the movements of the sun, marked by the solstices. Sometimes the equinoxes are marked as well, giving an eight armed wheel.
 

The Sun cross in its most simplified form (see right) is known in Northern Europe as Odin's cross, after the Chief God of the Norse pantheon. It is often used as an emblem by Asatruar, followers of the Norse religion. The word "cross" itself comes from the Old Norse word for this symbol: kros.