The Celtic-speaking lands of the western European edge, Ireland, Britain, and Gaul, hold a religious history marked by a difficult ancient record and a rich later imagination. Of the pre-Roman religion of the Druids and the Celtic gods, little survives from the inside: the Druids left no scriptures, and most of what is reported comes from hostile Roman observers like Julius Caesar, who described their priesthood, their sacred groves, their teaching of the soul's immortality, and (whether accurately or as war propaganda) their human sacrifices.
The richest windows are the medieval Irish and Welsh manuscripts, the Tain, the Mabinogi, written down centuries after Christianization by monks, preserving a mythology of gods, otherworlds (Tir na nOg, Avalon), and heroes in Christian dress. Ireland's conversion, associated with Patrick and Brigid, produced a distinctive monastic Christianity that became a beacon of learning in the early Middle Ages. In the modern era this same ground became the seedbed of a great deal of revived and invented tradition: the eighteenth- and nineteenth-century revival of Druidry, and the twentieth-century emergence of Wicca and Thelema, drew on real and imagined Celtic pasts. The region thus carries a double religious weight, an ancient tradition known chiefly through its enemies and its Christianized retellings, and a modern tradition of Pagan revival that looks back to it.