Before the Romans arrived in Britain, Celtic natives followed druidry. Little is known about this belief system, but the druids were spiritual leaders, keepers of tribal history and genealogy, keen astronomers, and administrators of the law. They were a highly trained, elite group, educated over a period of perhaps twenty years. High-ranking households included a resident bard, bardship being one of the various levels of druidry.
The Celts were animists. Nature was revered—all creatures, the earth, the skies, stars, water—were considered to be living and have a soul. Every pond, river, tree, rock, the wind and so on, had a spirit to be revered or beseeched. The four elements, earth, air, fire and water, were considered especially sacred. As most deities were localized, no unified pantheon developed. A few god/desses were more universal, but names are rare because it was believed to say a deity’s name out loud could take way the god/dess’s power. Rituals were usually held in groves. Festivals celebrated seasonal changes, harvests, and fertility.The Celtic Otherworld, Annwn, is mentioned in early Arthurian poems.
When the Romans sought to conquer Britain, controlling the people was one of their first objectives. They had discovered during the earlier subjugation of the Celts in Gaul, that wiping out the druids’ influence allowed them to inflict their own on the population. The Romans cited that druids practiced human sacrifice, but this may have been propaganda intended to further pry loose trust in the druids. The Romans had persecuted early Christians in the same manner.
In spite of the Romans’ brutality against the Celts’ spiritual leaders, the Celts were allowed to continue their native ways of worship. Roman deities often overlapped Celtic gods and were combined. In the fourth century, Christianity was adopted as the Romans’ official religion and was very slowly introduced into Britain. For a brief time after the occupation ended, native religion revived, indicated by fairly new temples dedicated to the old gods.
It was not until the fifth century, with the advent of monasticism, that Christianity truly took hold. Most Medieval Arthurian literature portrays Arthur as a devout Christian. However, it should be remembered that most Medieval writers were clerics of some degree, and people in general during those times were very deeply influenced by the church. It may be likely that Arthur truly was Christian, but in practicality, for him to have united Britain enough to turn the Saxons out, he would have had to please all his under-kings, nobles and the general population. Religion, one of those heavily argued subjects, would not necessarily be the cement that held his kingdom together.