Ancient Celtic religion, commonly known as Celtic paganism,[1][2][3] comprises the religious paganism,[1][2][3] comprises the religious beliefs and practices adhered to by the Iron Age people of Western Europe now known as the Celts, roughly between 500 BCE and 500 CE, spanning the La Tène period and the Roman era, and in the case of the Insular Celts the British and Irish Iron Age. Very little is known with any certainty about the subject, and apart from documented names that are thought to be of deities, the only detailed contemporary accounts are by hostile and probably not-well-informed Roman writers.

Model reconstructing the Pillar of the Boatmen in the Musée de Cluny, Paris. After 14 AD.

Celtic paganism was one of a larger group of Iron Age polytheistic religions of the Indo-European family. It comprised a large degree of variation both geographically and chronologically, although "behind this variety, broad structural similarities can be detected"[4] allowing there to be "a basic religious homogeneity" among the Celtic peoples.[5]

The Celtic pantheon consists of numerous recorded theonyms, both from Greco-Roman ethnography and from epigraphy. Among the most prominent ones are Teutatis, Taranis and Lugus. Figures from medieval Irish mythology have also been interpreted as iterations of earlier pre-Christian Insular deities in the study of comparative mythology.

According to Greek and Roman accounts, in Gaul, Britain and Ireland, there was a priestly caste of "magico-religious specialists" known as the druids, although very little is definitely known about them.[6] Following the Roman Empire's conquest of Gaul (58–51 BCE) and southern Britannia (43 AD), Celtic religious practices began to display elements of Romanisation, resulting in a syncretic Gallo-Roman culture with its own religious traditions with its own large set of deities, such as Cernunnos, Artio, Telesphorus, etc.

In Roman Britain this lost at least some ground to Christianity by the time the Romans left in 410, and in the next century began to be replaced by the pagan Anglo-Saxon religion over much of the country. Christianity had resumed missionary activity by the later 5th and the 6th centuries, also in Ireland, and the Celtic population was gradually Christianized supplanting the earlier religious traditions. However, polytheistic traditions left a legacy in many of the Celtic nations, influenced later mythology, and served as the basis for a new religious movement, Celtic Neopaganism, in the 20th century.