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Ancient Celtic religion, commonly known as Celtic paganism, was the religion of the ancient Celtic peoples of Europe. Because there are no extant native records of their beliefs, evidence about their religion is gleaned from archaeology, Greco-Roman accounts (some of them hostile and probably not well-informed), and literature from the early Christian period. Celtic paganism was one of a larger group of polytheistic Indo-European religions of Iron Age Europe.

While the specific deities worshipped varied by region and over time, underlying this were broad similarities in both deities and "a basic religious homogeneity" among the Celtic peoples. Widely worshipped Celtic gods included Lugus, Toutatis, Taranis, Cernunnos, Epona, Maponos, Belenos, Ogmios, and Sucellos. Sacred springs were often associated with Celtic healing deities. Triplicity is a common theme, with a number of deities seen as threefold, for example the Three Mothers.

The druids were the priests of Celtic religion, but little is definitively known about them. Greco-Roman writers said the Celts held ceremonies in sacred groves and other natural shrines, called nemetons, while some Celtic peoples also built temples or ritual enclosures. Celtic peoples often made votive offerings which would be deposited in water and wetlands, or in ritual shafts and wells. There is evidence that ancient Celtic peoples sacrificed animals, almost always livestock or working animals. There is some evidence that ancient Celts sacrificed humans, and Caesar in his accounts of the Gallic wars claims that the Gauls sacrificed criminals by burning them in a wicker man, though this may have been imperial propaganda.