The Celts of the ancient world believed that many spirits and divine beings inhabited the world around them, and that humans could establish a rapport with these beings. The archaeological and the literary record indicate that ritual practice in Celtic societies lacked a clear distinction between the sacred and profane; rituals, offerings, and correct behaviour maintained a balance between gods, spirits and humans and harnessed supernatural forces for the benefit of the community. The pagan Celts perceived the presence of the supernatural as integral to, and interwoven with, the material world. Every mountain, river, spring, marsh, tree and rocky outcrop was inspirited. While the polytheistic cultures of ancient Greece and Rome revolved around urban life, ancient Celtic society was predominantly rural. The close link with the natural world is reflected in what we know of the religious systems of Celtic Europe during the late 1st millennium BCE and early 1st millennium CE.
As in many polytheistic systems, the local spirits honoured were those of both the wild and cultivated landscapes and their inhabitants. The ancient Celts venerated the spirits who inhabited local mountains, forests and springs. Certain animals were seen as messengers of the spirits or gods. In Tribal territories, the ground and waters which received the dead were imbued with sanctity and revered by their living relatives. Sanctuaries were sacred spaces separated from the ordinary world, often in natural locations such as springs, sacred groves or lakes. Many topographical features were honoured as the abodes of powerful spirits or deities, with geographical features named for tutelary deities. Offerings of jewellery, weapons or foodstuffs were placed in offering pits and bodies of water dedicated to these beings. These offerings linked the donor to the place and spirits in a concrete way. The spirits of watery places were honoured as givers of life and as links between the physical realm and the other world.
In Ireland, the tutelary goddesses Boann and Sionnan give their names to the rivers Boyne and Shannon, and the tales of these goddesses are the origin stories of the rivers themselves. The threefold goddess Brighid is associated with a number of holy wells and The Morrígan is connected with the River Unius. There is abundant evidence for the veneration of water by the Celts and indeed by their Bronze Age forebears. In the Pre-Roman Iron Age, lakes, rivers, springs and bogs received special offerings of metalwork, wooden objects, animals and, occasionally, of human beings. Popular shrines were deemed to be under the care of a local guardian spirit whose worshippers would perform daily rituals.
The spirit would be ceremonially evoked to appear in different animal forms or as the goddess herself, in the form of maiden, mother or old crone as the mood took her. The Celts and Druids, held trees very sacred especially the Oak and was an important symbol in their religion. They believed that the Oak represented the World Tree, which ties all three worlds together. Its highest branches reach into the Upper world, their trunks lived in the Earth world and below its roots were a passageway into the Underworld. The Druids also revered the Oak as a steadfast and hearty tree that represented the immortal soul.