Celtic polytheism, sometimes known as Celtic paganism, refers to the religious beliefs and practises of the ancient Celtic peoples of western Europe prior to Christianisation.

Celtic polytheism, as its name suggests, was polytheistic, believing in a number of different deities, and was also animistic, believing in spirits existing in natural objects such as trees and rocks. Religious beliefs and practises of the Celts varied throughout the different Celtic lands, which included Ireland, Britain, Celtiberia, Gaul, areas along the Danube river, and Galatia; however there were commonalities shared by all.

Celtic religious practices bear the marks of Romanization following the Roman Empire's conquest of certain Celtic lands such as Gaul (58–51 BCE) and Britain (43 CE), although the depth and significance of Romanization is a subject of scholarly disagreement.

Celtic polytheism declined in the Roman Empire period, especially after the outlawing of one form of it, Druidism, by the emperor Claudius in 54 CE. It persisted somewhat longer in Britain and Ireland, where it gradually disappeared during Christianization, over the 5th to 6th centuries.

Celtic polytheists probably did not have a name for their religion until faced with other religions to compare it to. Therefore, the only titles bestowed upon Celtic religion are the ones which were used to describe the religion in a competitive manner, such as the Latin word "paganism".

We know comparatively little about Celtic polytheism because the evidence for it is fragmentary, largely due to the fact that the pagan Celts themselves wrote nothing down about their religion. ^[1][2] Therefore all we have to study their religion from is the literature from the early Christian period, commentaries from classical Greek and Roman scholars, and archaeological evidence.^[3]

The Iron Age Celts wrote nothing down about their religion not because they couldn't (many knew the Greek alphabet, and used it for other purposes^[2] and the Celts of the British Isles also made use of their native Ogham script^{[citation needed]}), but because it was forbidden. The Druids, or priestly caste of the Celts, would only allow their knowledge to be passed orally, possibly so as to protect its secrets from outsiders.^[2]

Various Greek and Roman writers of the ancient world commented on the Celts and their beliefs. The Roman general (and later dictator) Julius Caesar, when leading the conquering armies of the Roman Republic against Celtic Gaul, made various descriptions of the inhabitants, though some of his claims, such as that the Druids practised human sacrifice by burning people in wicker men, have come under scrutiny by modern scholars.

However, the key problem with the use of these sources is that they were often biased against the Celts, whom the classical peoples viewed as "barbarians".^[1] In the case of the Romans who conquered several Celtic realms, they would have likely been biased in favour of making the Celts look uncivilised, thereby giving the "civilised" Romans more reason to conquer them. ^[4]

The other literary sources come from the Celtic lands of Wales and Ireland, in the Christian mediaeval period, long after the decline of Celtic paganism. These sources are in the form of mythological stories, such as the Welsh Mabinogion and the Irish vernacular sources such as the Táin Bó Cúailnge. These were initially written in the Welsh and Irish language respectively.

They were written several centuries after Christianity became the dominant religion in these regions, and were written down by Christian monks (at the time, monks would have been some of the few people with the ability to write). Instead of treating the characters as deities, they are allocated the roles of being historical heroes, for instance, in the Irish sources the gods are claimed to be an ancient tribe of humans known as the Tuatha Dé Danann. Because they were written in a very Christian context, these sources must be scrutinized with even more rigour than the classical sources in assessing their validity as evidence for pagan Celtic religion.^[1]

While it is possible to single out specific texts which – because of their pagan content – can be strongly argued to encapsulate genuine echoes or resonances of the pre-Christian past, opinion is divided as to whether these texts contain substantive material derived from oral tradition as preserved by bards or whether they were the creation of the mediaeval monastic tradition.^[1]

The archaeological evidence does not contain the bias inherent in the literary sources. Nonetheless, our interpretation of this evidence can sometimes be coloured by the 21st century mindset.^[1]

Various archaeological discoveries have aided our understanding of the pagan religion of the Celts. One is the minted coins of Gaul, Raetia, Noricum, and Britain, and another is the sculptures, monuments, and inscriptions associated with the Celts of continental Europe and of Roman Britain. Most of the monuments, and their accompanying inscriptions, belong to the Roman period and reflect a considerable degree of syncretism between Celtic and Roman gods; even where figures and motifs appear to derive from pre-Roman tradition, they are difficult to interpret in the absence of a preserved literature on mythology. A notable example of this is the horned deity that was called Cernunnos; we have found several depictions and inscriptions of him, but know very little about the myths that would have been associated with him or how he was worshipped.

Celtic religion was polytheistic, believing in many deities, both gods and goddesses. The most notable of these were pan-Celtic, being worshipped across much of the Celtic world, albeit under various regional names and with different associations. Despite the notability of these pan-Celtic deities, they make up only a tiny percentage of Celtic gods; out of the roughly 300 Celtic deities that we know about, only around 60 can be found in more than one region, and of those, only about 20–30 are pan-Celtic ^[5].

The Celts were also animists, believing in deities existing in most aspects of nature, such as in trees and streams, who were often venerated at local shrines.

According to classical era sources, the Celts worshipped the forces of nature and did not envisage deities in anthropomorphic terms,^[6] as other pagan peoples such as the Greeks, Romans, and Egyptians did. This appeared to change as the classical peoples grew in influence over the Celtic cultures, as the Celts did begin to give their deities human forms, and they moved from a more animistic-based faith to a more Romanized polytheistic view.

Several of these deities, including Lugus and Matrones, exhibited triplism, being found in a set of three^[7].

Insular Celts swore their oaths by their personal or tribal gods, and the land, sea and sky; as in, "I swear by the gods by whom my people swear" and "If I break my oath, may the land open to swallow me, the sea rise to drown me, and the sky fall upon me."^[8]

Some deities of the Celts were deities of major natural occurrences, such as the sun. The Celts did not worship the sun, but saw it as a symbol for that aspect of divinity. These deities were generally worshipped across the Celtic lands, however, they often went under different names. An example of this was the god Lugus, who appeared in later Irish mythology as Lugh, and in later Welsh mythology, where he appeared as Lleu Llaw Gyffes.

Another widespread pan-Celtic god was Taranis, a god of thunder, whose worship has been detected as having occurred in Gaul, Britain and Hispania. Other similar deities included Toutatis, a god of tribal protection in Gaul and Britain, Belenos, a god of healing, and Cernunnos, a horned figure found in Gaul.

There were also pan-Celtic goddesses. Examples of this include a mother goddess (such as Danu from Ireland and Dôn from Wales), a goddess of water (such as Sulis), and a goddess of horses (such as Epona in France, Macha in Ireland, and Rhiannon in Wales).

When the Romans conquered the Celtic lands of Gaul, Hispania and Britain, they equated the Celtic gods with their own deities. For instance, they claimed that the Gaulish Celtic god Belenos was the same as their own god Apollo, and that Lugus was the same as their own Mercury. These Roman descriptions comparing Celtic and Roman deities are one of the few sources of literary information that we have about the Celtic gods.

The Celts were animists, believing that all aspects of the natural world contained spirits, and that these spirits could be communicated with.^[9].

These animistic deities were often worshipped, so places such as rocks, streams, mountains, and trees may all have had shrines or offerings devoted to a deity residing there. A similar belief is found in modern Shinto in Japan, through the belief of kami. These would have been local deities, known and worshipped by inhabitants living near to the shrine itself, and not pan-Celtic like some of the polytheistic gods.

Among the most popular sites for the veneration of animistic deities were trees; the oak, ash, and thorn were considered to be the most sacred. The early Celts considered some trees to be sacred. The importance of trees in Celtic religion is shown by the fact that the very name of the Eburonian tribe contains a reference to the yew tree, and that names like Mac Cuilinn (son of holly) and Mac Ibar (son of yew) appear in Irish myths. In Ireland, wisdom was symbolized by the salmon who feed on the hazelnuts from the trees that surround the well of wisdom (Tobar Segais).

Hot springs and rivers were also popular sites for worship, and were commonly associated with healing.

One of the most popular theories for a belief in fairies (such as knockers, clurichaun, and pixies^[10]) in Christianised Celtic areas is that they were a recurring folk belief of these animistic deities, placed under a Christian worldview, where they were seen no longer as nature deities but as malevolant spirits. Sometimes these fairies were treated just the same as previous pagan nature gods had been, with offerings being placed on trees and other shrines to both placate them from committing negative actions and ensuring a good harvest or hunt etc.

There is no direct information that has survived on what the Celts believed happened after death. However, from archaeological discoveries, Roman accounts, and later mythology, possible ideas of a Celtic afterlife can be established.

Celtic burial practices, which included burying food, weapons, and ornaments with the dead, suggest a belief in life after death.^[11]

The druids, the Celtic learned class which included members of the clergy, were said by Caesar to have believed in reincarnation and transmigration of the soul along with astronomy and the nature and power of the gods.^[12]

A common factor in later mythologies from Christianised Celtic nations was the otherworld.^[13] This was the realm of the fairy folk and other supernatural beings, who would entice humans into their realm. Sometimes this otherworld was claimed to exist underground, whilst at other times it was said to lie far to the west. Several scholars have suggested that the otherworld was the pagan Celtic afterlife,^[13] though there is no direct evidence to prove this.

Virtually everything that we know about the pagan Celtic religious festivals come from insular sources from Ireland. The Celts of these lands practised four religious festivals a year. ^[2] These festivals were equidistant from each other, and divided the year into four quarters.

The first festival was Samhain (Calan Gaeaf in Wales), held on November 1st. It marked the end of one pastoral year, and the beginning of another, and was similarly thought of as the time when spirits of the Otherworld became visible to humans^[2]. It was Christianised as All Hallows Day or The Day of the Dead the Eve of which is Halloween, which has kept its associations with spirits and the supernatural right into the contemporary period.

The second festival was Imbolc (Gŵyl Fair y Canhwyllau or Gŵyl y Canhwyllau in Wales), celebrated on the eve of February 1. It was sacred to the fertility goddess Brigit, and as such was a spring festival. It was later Christianised as the feast of St Brigid.^[2] The French scholar Joseph Vendryes compared it to the Roman lustrations^{[citation needed]}.

The third festival was Beltaine (Calan Mai in Wales), held on the eve of May 1st. It was devoted to the god Bel, and a common practice was the lighting of fires. It was later Christianised as the feast of St John the Baptist,^[2] and the festival of May Day is generally thought to have been based upon it.

The fourth festival was Lughnasadh (Calan Awst in Wales), which took place in August. It revolved around the god Lugh, who, according to mythology, was giving a feast for his foster mother Tailtu at that time. ^[2]

Classical sources claimed that the Celts had no temples (before the Gallo-Roman period) and that their ceremonies took place in forest sanctuaries. However, archaeologists have discovered a large number of temple sites excavated throughout the Celtic world, primarily in Gaul. In the Gallo-Roman period, more permanent stone temples were erected, and many of them have been discovered by archaeologists in Britain as well as in Gaul. Indeed, a distinct type of Celto-Roman temple called a fanum also was developed. This was distinguished from a common Roman shrine by having an ambulatory on all four sides of the central cella.

Celtic religious practice was probably sacrificial in its interactions with the gods. Roman writers stated that the Celts practiced human sacrifice in Gaul: Cicero, Julius Caesar, Suetonius, and Lucan all refer to it, and Pliny the Elder says that it occurred in Britain, too. It was forbidden under Tiberius and Claudius. However there is also the possibility that these claims may have been false, and used as a sort of propaganda to justify the Roman conquest of these territories. There are only very few recorded archaeological discoveries which preserve evidence of human sacrifice and thus most contemporary historians tend to regard human sacrifice as rare within Celtic cultures. There is some circumstantial evidence that human sacrifice was known in Ireland and was later forbidden by St. Patrick, a claim which has also been disputed.

There was also a warrior cult that centered on the severed heads of their enemies. The Celts provided their dead with weapons and other accoutrements, which indicates that they most likely believed in some form of an afterlife.

An example of a ceremony was the ritual of the oak and the mistletoe.

According to Poseidonius and later classical authors Gaulish religion and culture were the concern of three professional classes—the druids, the bards, and the vates. This threefold hierarchy had its reflection among the two main branches of Celts in Ireland and Wales, but is best represented in early Irish tradition with its draoithe (druids), filidh (visionary poets), and Faidh (seers). However these categories are not always fixed, and may be named or divided differently in different primary sources.

A Druid was a member of the learned class among the ancient Celts. They acted as priests, teachers, and judges. The earliest known records of the Druids come from the 3rd century BCE. Some scholars have suggested that the Druids were the Celtic counterparts of the Brahmins of India.

In Ireland the filid were visionary poets, associated with lorekeeping, versecraft, and the memorization of vast numbers of poems. They were also magicians, as Irish magic is intrinsically connected to poetry, and the satire of a gifted poet was a serious curse upon the one being satirized. To run afoul of a poet was a dangerous thing indeed to a people who valued reputation and honor more than life itself.

In Ireland a "bard" was considered a lesser grade of poet than a fili - more of a minstrel and rote reciter than an inspired artist with magical powers. However, in Wales bardd was the word for their visionary poets, and used in the same manner fili was in Ireland and Scotland.

The Celtic poets, of whatever grade, were composers of eulogy and satire, and a chief duty was that of composing and reciting verses on heroes and their deeds, and memorizing the genealogies of their patrons. It was essential to their livelihood that they increase the fame of their patrons, via tales, poems and songs. As early as the 1st century CE, the Latin author Lucan referred to "bards" as the national poets or minstrels of Gaul and Britain. In Gaul the institution gradually disappeared, whereas in Ireland and Wales it survived. The Irish bard through chanting preserved a tradition of poetic eulogy. In Wales, where the word bardd has always been used for poet, the bardic order was codified into distinct grades in the 10th century. Despite a decline of the order toward the end of the European Middle Ages, the Welsh tradition has persisted and is celebrated in the annual eisteddfod, a national assembly of poets and musicians.