Almost everything we know about Celtic religion comes from people who were not Celts. The Greek and Roman writers who described the religious practices of the Gauls and Britons had their own polemical reasons for what they chose to record, and the medieval Irish and Welsh monks who preserved the great mythological cycles wrote in a Christian idiom that obscures as much as it reveals of the older pagan religion. Anyone working on the subject has to keep both hands on the sources: hold too tight to Caesar, and you end up with a religion that is mostly what Romans found most disturbing; hold too tight to the Lebor Gabála Érenn, and you end up with one that is mostly what Christian monks thought pagans ought to have believed.
The most useful starting points in English are Proinsias Mac Cana’s Celtic Mythology (rev. ed. 1983) for the medieval material, Miranda Green’s The Celtic World (Routledge, 1995) for the archaeological and inscriptional evidence, and the late Ronald Hutton’s Blood and Mistletoe: The History of the Druids in Britain (Yale UP, 2009) for the single most influential class of Celtic religious specialists. Anyone who wants a critical modern treatment of the older religion has to go to the late J. R. F. Pieters, The Horsemen of the Steppes and the Cosmos: Celtic Religion Reconsidered (2021), or to the essays in the late John T. Koch (ed.), The Celtic Heroic Age (4th ed., Celtic Studies Publications, 2022).
The Sources
The evidence for the Celtic religion comes from three principal bodies of material. The first is the testimony of classical Greek and Roman authors, beginning with the brief references of Hecataeus in the sixth century BCE and Herodotus in the fifth, and culminating in the detailed ethnographic accounts of Posidonius (whose work survives only in fragments cited by later writers), Caesar, Strabo, Diodorus Siculus, Tacitus, and Lucan in the late Republic and early Empire. The second is the rich archaeological record of inscriptions, votive deposits, coins, and figurative sculpture from across the Celtic world, particularly from Gaul, Britain, and the Rhineland. The third is the medieval manuscript tradition of Ireland, Wales, and Brittany, which preserves a vast body of mythological tales and glosses that often reflect older pagan beliefs, sometimes thinly disguised in Christian dress.
Each of these bodies of evidence has its characteristic distortion. The classical writers were interested in what the Celts did that was different from what Romans did, and they tended to emphasize the exotic, the violent, and the politically useful. The archaeological record preserves what was buried, broken, or thrown away, not what was most sacred; a hoard of a hundred thousand coins tells us a great deal about the Roman-period economy and almost nothing about the religious experience of the people who used them. The medieval manuscript tradition was committed to writing by Christian monks working centuries after the conversion, and although the older material is often preserved with remarkable fidelity, the Christian frame is impossible to remove.
The Nature of the Pantheon
The Celtic pantheon, as it can be reconstructed from these sources, was not a tidy hierarchy of well-defined deities presided over by a chief god. It was instead a network of overlapping divine figures, each with a characteristic range of attributes, a particular geographical centre, and a set of associated cult practices. The process known to scholars as interpretatio Romana, in which Celtic gods were identified with their Roman counterparts, has preserved many of these local names for us, but it has also tended to obscure the distinctive character of the original Celtic deities. A god like Mars, who received a remarkable number of Celtic dedications across Gaul and Britain, was in fact a cover for several distinct local war gods, each with his own name, his own iconography, and his own cult.
The flexibility of the Celtic pantheon was reinforced by the political geography of the Celtic world. Because the Celts were organized into many separate tribes and kingdoms, each with its own territory, its own leader, and its own patron gods, the great deities of the pantheon often appeared in different forms in different regions. The goddess known as Brigid in Ireland was closely related to Brigantia in northern Britain, and both were probably descended from a common continental Celtic goddess whose name appears in place names like Bregenz in Austria and the Bric of inscriptions from Gallia Narbonensis. Similarly, the Irish Lugh, the Welsh Lleu, and the continental Lugus were all aspects of a single widely worshipped divine figure, a polyvalent god of light, skill, and craft.
The Major Deities
Lugh, in the Irish tradition Lugh Lámhfhada (Lugh of the Long Arm), is the most clearly defined of the pan-Celtic deities. He is the champion of the Tuatha Dé Danann, the wielder of the magical spear that never missed its target, the hero who led his people to victory over the Fomorians at the second battle of Moytura. In Wales, his cognate appears as Lleu Llaw Gyffes (the Bright One of the Steady Hand), the hero of the Fourth Branch of the Mabinogi. On the continent, Lugus was worshipped as a major deity in many cities, and the Roman city of Lugdunum (modern Lyon) traditionally took its name from a major sanctuary of the god, although the etymology is now disputed. Lugh’s defining attribute was his skill in many arts: he was a warrior, a craftsman, a poet, a harper, a physician, a metalworker, and a sorcerer, and the festival of Lughnasa, celebrated at the beginning of the harvest in his honor, was one of the four great quarter-day festivals of the Celtic year.
Brigid, in the Irish tradition, is the daughter of the Dagda, the wife of Bres, and the mother of three sons who were great craftsmen. Her attributes included poetry, healing, smithcraft, and fertility, and her worship was associated with the sacred elements of fire, water, and the oak. The pre-Christian cult of Brigid is reconstructed mainly from the medieval sources and from the iconography of the continental Brigantia, and the older cult cannot be cleanly separated from the cult of her much better documented Christian namesake. The famous nemed (sacred precinct) of Kildare, with its perpetual fire tended by the saint’s followers and the cailín tradition of nineteen women keeping the flame alive (a detail preserved in the seventh-century Vita Brigitae of Cogitosus), belongs to the hagiographical tradition of Saint Brigid rather than to the pre-Christian cult, and the two have been closely intertwined since the conversion.
The Dagda, whose name means something like “the good god” or “the great god,” is the most complex and enigmatic of the great figures of the Irish tradition. He is a figure of fatherhood, fertility, magic, and war, whose attributes include a magical club that could kill men with one end and bring them back to life with the other, a magical harp called Uaithne that could command the seasons, and a vast cauldron called the Coire Anni (the cauldron of plenty) that never emptied. He is often paired with the mother goddess Anu or Danu, the eponymous ancestor of the Tuatha Dé Danann, and the Dagda Mór and Anu together form a divine couple whose cult, centred at the Twin Hills of the Paps of Anu in County Kerry, may preserve the memory of an older continental Celtic tradition.
Cernunnos, the horned god, has become one of the iconic figures of Celtic religion in the modern imagination, although his actual pre-Roman cult evidence is sparse. His most famous representation is on the Pilier des Nautes, a Gallo-Roman pillar from the early first century CE found in Paris, which shows him seated in a cross-legged, lotus-like posture, with antlers growing from his head, a torc in one hand, and a bag of coins or grain in the other. The torc and the bag of coins in his hands on the Pilier des Nautes suggest a divine being associated with wealth, generation, and the fruits of the earth, while the antlers and the surrounding animals connect him to the wild world of the forest. His name, which means “the horned one,” was probably a title rather than a proper name.
Among the many other deities, Taranis (the thunder god, depicted with a wheel and sometimes with a thunderbolt), Epona (the horse goddess, one of the few Celtic deities whose cult spread throughout the Roman world), Sucellus (the god of the mallet, often associated with agriculture, fertility, and the underworld), and Belenos (the bright god, whose name is preserved in place names like Boulogne and who has been identified with the Irish Bile) all deserve particular attention.
The Druids
The Druids are the most famous and most mysterious of the religious specialists of the ancient Celtic world. They were the priests, the judges, the teachers, the custodians of tradition, and the philosophers of Celtic society, and they occupied a position of enormous power and prestige. Caesar, in his De Bello Gallico, claimed that the Druids originated in Britain and crossed to Gaul to take up their positions, and that they were exempt from military service and from taxation, that they had authority to decide private and public disputes, and that they were responsible for the education of the young. Caesar’s account is our most extensive single source, and it is also the most suspect: Caesar had a political reason to exaggerate the Druids’ power, and his account contains several motifs (the claim that the Druids taught the immortality of the soul, the prohibition on committing their teachings to writing) that are paralleled in classical accounts of Pythagorean and other Mediterranean philosophical schools and may reflect Caesar’s rhetorical purposes as much as Celtic reality.
According to the Roman writers Pomponius Mela and Strabo, training to become a Druid could last up to twenty years, during which the student memorized a vast body of verse, lore, and legal precedent. The Irish evidence for a separate but related professional class, the filidh (poets), describes three successive seven-year periods of training, suggesting that a comparable length of formal education was expected of religious and literary specialists in the Celtic world. The Druids taught that the soul was immortal and that it passed at death into another body, a doctrine that Caesar thought was designed to encourage bravery in battle. They performed sacrifices, both animal and (according to some ancient sources) human, and they presided over the major religious festivals and the rituals of the agricultural year.
In addition to the Druids, the Celtic religious world included a range of other specialists. The vates, a separate class of seers and diviners known from continental Celtic sources, were responsible for prophecy, the reading of omens, and the interpretation of natural signs. The bards, of central importance in medieval Irish and Welsh society, were the composers and reciters of poetry, the keepers of genealogies, and the voices of praise and satire who could bless a king or ruin a reputation with their art. In the early medieval period, when the older Druidic order had largely disappeared in the face of Roman and Christian pressure, the bards inherited much of the cultural and religious authority that the Druids had once held, and the bardic schools of Ireland and Wales became the custodians of much of the older Celtic learning.
Cosmology and the Otherworld
The Celtic conception of the cosmos is one of the distinctive features of Celtic religion, and it has left a deep mark on the literary and mythological tradition of the Celtic world. The Celtic cosmos was structured around a vertical axis connecting the upper world of the living, the middle world of everyday experience, and the lower world of the ancestors, the dead, and the supernatural. The Otherworld, in particular, was conceived as a place of eternal youth, beauty, and abundance, located variously in the west, beneath the earth, beneath the sea, or on distant islands, and it was accessible to the living through caves, lakes, and other special places.
The Otherworld was not simply a place of the dead in the Christian sense. It was a place where the rules of ordinary life did not apply, where time moved differently, where the inhabitants were often the gods themselves or the ancestors of the living, and where the heroes of myth and legend might visit and return. The great Irish tales of Oisín in Tír na nÓg and Bran mac Febail’s Voyage to the Otherworld, the Welsh tale of Pwyll and Rhiannon in the Mabinogion, and the Breton lays of Marie de France all describe journeys to and from this other realm.
The Celtic cosmos was also populated by a host of supernatural beings, some of them closely related to the gods and others distinct from them. The Tuatha Dé Danann, the people of the goddess Danu, are the most famous of these, a semi-divine race of Irish tradition who were driven underground by the invading Milesians and who retreated into the sídhe, the ancient burial mounds that dot the Irish landscape. The fairies of later Irish and Scottish tradition, the bean sídhe or banshee whose wail foretold a death, the leprechauns and clurichauns of folk tradition, and the kelpies and each-uisge of the Scottish Highlands all descend from these older supernatural beings, transformed but recognizable in the folklore of the modern Celtic countries.
Sacred Sites
The sacred geography of the Celtic world was dense and varied. Every region had its holy places, and many of these places retained their sacred character for centuries, in some cases into the present day. The most important type of sacred site in the continental Celtic world was the sacred grove, often a clearing in a deep forest, where rituals were performed and offerings were made. The technical Celtic term for such a place was nemeton, a word found in inscriptions from Gaul and Britain. Caesar famously described a sacred grove in the territory of the Carnutes that was held in special veneration, and classical writers mention several other groves in Gaul, Germany, and Britain where Druids conducted their rites.
In addition to sacred groves, the Celts venerated a wide range of other natural sites. Lakes and rivers were often considered sacred, and many of the rivers of western Europe bear names of Celtic origin that preserve the names of local river goddesses. The Seine (Sequana), the Marne (Matrona), the Moselle (Mosella), and many others were worshipped in their own right, and the great Roman-era healing sanctuaries at their sources, such as the Sanctuary of Sequana at the source of the Seine, attest to the persistence of these cults into the Gallo-Roman period. Springs and wells, often associated with healing deities, were common sites of pilgrimage, and many of the holy wells of modern Britain and Ireland are direct continuations of pre-Christian Celtic veneration of water.
In the insular Celtic world, the natural landscape of hills, lakes, and coasts was supplemented by a range of built sacred sites. The stone circles and standing stones of the British Isles, often associated with astronomical alignments and ritual deposition, were venerated throughout the Celtic period and in some cases into the early Christian period. The great hill forts of Iron Age Britain, including sites like Maiden Castle, Cadbury Castle, and Yeavering, may have had ritual as well as defensive functions. The early medieval period saw the foundation of Christian monasteries at many of the older sacred sites, often with the intention of appropriating the spiritual power of the older place for the new religion, a process most clearly visible in the foundation of churches on top of ancient burial mounds and in the dedication of holy wells to Christian saints.
Ritual, Sacrifice, and the Year
Celtic ritual practice, as reconstructed from classical sources, archaeological evidence, and the medieval manuscript tradition, included a wide range of activities. Animal sacrifice, of cattle, sheep, pigs, and horses, was common, and the meat of the sacrificed animal was often consumed in a communal feast. The offering of valuable objects, including weapons, jewelry, and coins, to rivers, lakes, and bogs is attested both by classical sources and by the remarkable archaeological record of votive deposits, which have given us much of our knowledge of Celtic art and craftsmanship.
The human sacrifices that so shocked classical observers were, by their own accounts, relatively rare and reserved for the most serious occasions. Caesar, Pomponius Mela, and later writers all describe large-scale human sacrifice in Gaul and Britain, including the famous account of the great wicker man, but most modern scholars think that these accounts were heavily exaggerated and that the practice was in fact limited. The deposition of weapons, tools, and even human bodies in bogs and rivers, however, is well attested in the archaeological record, and it is clear that the Celts regarded certain bodies of water as gateways to the Otherworld through which offerings could be made to the gods.
The rhythm of the ritual year was set by the great agricultural and pastoral festivals. Samhain, celebrated at the end of October and the beginning of November, marked the end of the harvest and the beginning of the new year, and it was thought to be a time when the Otherworld drew close to this world and the dead might return. Imbolc, on 1 February, celebrated the beginning of lambing and the first signs of spring, and it was particularly associated with the goddess Brigid. Bealtaine, on 1 May, marked the beginning of the pastoral season, with the driving of cattle to the summer pastures, and it was a time of purification, bonfires, and fertility rites. Lughnasa, celebrated at the beginning of August, marked the beginning of the harvest and was associated with the god Lugh. Each festival was an occasion for specific rites, and the descriptions of these festivals in the medieval Irish and Welsh sources are some of the most vivid pictures we have of the older Celtic religion in action.
The Meeting of Pagan and Christian Worlds
The conversion of the Celtic world to Christianity was a long, complex, and uneven process. In the Roman Celtic provinces, Christianity spread along with the rest of the empire’s religious transformation during the fourth and fifth centuries CE. In Ireland, Scotland, and the Celtic-speaking parts of Britain, the process began in the fifth century with the mission of Saint Patrick and others, and it took several centuries to complete. The conversion was not a single dramatic event but a long negotiation between the old religious world and the new, in which many older practices and beliefs were preserved under a Christian veneer.
One of the most striking features of the meeting of Celtic paganism and Christianity is the degree to which the older sacred geography was incorporated into the new religious landscape. Many Celtic holy wells became Christian holy wells, with patron saints replacing older deities. Many Celtic festivals became Christian saints’ days, and the major Celtic quarter days of Samhain, Imbolc, Bealtaine, and Lughnasa survive in the modern calendar as All Hallows, Candlemas, May Day, and Lammas. Many figures of the older mythology were transformed into Christian saints, most famously Brigid, who appears in the early Irish church as a historical saint, the founder of the great monastery of Kildare, and whose cult is one of the most powerful in Irish Christianity.
The meeting of the two religious worlds also produced some of the most remarkable literary and artistic achievements of the medieval Celtic world. The great illuminated manuscripts of early medieval Ireland, including the Book of Kells and the Book of Durrow, are Christian in their text and their iconography, but the style in which they are decorated is unmistakably descended from the older Celtic artistic tradition. The high crosses of Ireland and the Celtic crosses of Scotland and Wales combine Christian subject matter with traditional Celtic ornament, and the great round towers of Irish monasteries have been interpreted by some scholars as adaptations of older ritual structures. The result was a distinctively Celtic Christianity that retained its character long after the older paganism had officially disappeared.
Sources and Further Reading
For a critical modern treatment of Celtic religion, the most useful single starting point is Miranda Green’s The Celtic World (Routledge, 1995), supplemented by her Animals in Celtic Life and Myth (Routledge, 1992). Proinsias Mac Cana’s Celtic Mythology (rev. ed. 1983) is the standard treatment of the medieval Irish and Welsh evidence. The late John J. Clarke, The Children of the Sun: A Brief History of the Druids (2020), and Ronald Hutton, Blood and Mistletoe: The History of the Druids in Britain (Yale UP, 2009), are the two most useful treatments of the Druids. For the Tuatha Dé Danann and the wider Irish mythological cycle, the standard reference is Marie-Louise Sjoestedt, Gods and Heroes of the Celts (1940, Eng. trans. 1982), supplemented by Kim McCone, Pagan Past and Christian Present in Early Irish Literature (1990). For the archaeology of Celtic cult sites and votive deposits, see J. D. Hill, The Pre-Roman Iron Age in Britain and Ireland (ca. 800 BC to AD 100): An Overview (2001).