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Archaeological sources

The archaeological evidence does not contain the bias inherent in the literary sources. Nonetheless, the interpretation of this evidence can be coloured by the 21st century mindset. Various archaeological discoveries have aided understanding of the religion of the Celts.

Most surviving Celtic art is not figurative; some art historians have suggested that the complex and compelling decorative motifs that characterize some periods have a religious significance, but the understanding of what that might be appears to be irretrievably lost. Surviving figurative monumental sculpture comes almost entirely from Romano-Celtic contexts, and broadly follows provincial Roman styles, though figures who are probably deities often wear torcs, and there may be inscriptions in Roman letters with what appear to be Romanized Celtic names. The Pillar of the Boatmen from Paris, with many deity figures, is the most comprehensive example, datable by a dedication to the Emperor Tiberius (r. from 14 AD).

Monumental stone sculptures from before conquest by the Romans are much more rare, and it is far from clear that deities are represented. The most significant are the Warrior of Hirschlanden and " Glauberg Prince" (respectively 6th and 5th-century BC, from Germany), the Mšecké Žehrovice Head (probably 2nd-century BC, Czech Republic), and sanctuaries of some sort at the southern French oppida of Roquepertuse and Entremont. There are also a number of Celtiberian standing "warrior" figures, and several other stone heads from various areas. In general, even early monumental sculpture is found in areas with higher levels of contact with the classical world, through trade. It is possible that wooden monumental sculpture was more common. Small heads are more common, mainly surviving as ornament in metalwork, and there are also animals and birds that may have a religious significance, as on the Basse Yutz Flagons. The Strettweg Cult Wagon is probably associated with libations or sacrifices, and pairs of metal "spoons" probably used for divination have been found.

Celtic coinage, from the late 4th century BC until conquest, clearly copies Greek and Roman examples, sometimes very closely, but the heads and horses that are the most popular motifs may have a local religious significance. There are also the coins of the Roman provinces in the Celtic lands of Gaul, Raetia, Noricum, and Britannia.

Most of the surviving 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 god that was called Cernunnos; several depictions and inscriptions of him have been found, but very little is known about the myths that would have been associated with him or how he was worshipped.