No Celtic community left a scripture, so its religion must be read from three imperfect witnesses: archaeology, the reports of Greek and Roman authors, and medieval literature written down centuries after conversion. The archaeology shows votive deposits of weapons and treasure in rivers and bogs, sanctuaries such as Gournay-sur-Aronde in Gaul, and images like the antlered figure on the Gundestrup cauldron usually identified with Cernunnos. Caesar and other classical writers describe the druids as a learned class of priests, judges and teachers who committed their doctrine to memory rather than writing, taught the immortality of the soul, and presided over sacrifice; Ronald Hutton's work shows how thin and hostile that testimony is, and how much later ages built upon it.
The gods are best known from Ireland and Wales. Lugh of the many skills, Brigid of poetry, healing and smithcraft, the Dagda with his club and cauldron, and the battle goddess the Morrigan appear in Irish cycles as the Tuatha De Danann, the people of the goddess Danu, euhemerized by Christian scribes into a race of wondrous predecessors. Gaulish dedications name hundreds of local deities, with Epona the horse goddess carried as far as Rome itself.
The insular year turned on four festivals, Samhain, Imbolc, Beltane and Lughnasadh, marking the pastoral seasons; their names and customs survived Christianization as saints' days and folk observances. The Otherworld, reached through mounds, mists or the western sea, appears in tales of Tir na nOg, Annwn and Mag Mell as a land without age or death.