The pre-Christian religion of the Basque country is documented thinly and late, mostly from folklore collected in the nineteenth and twentieth centuries, notably by Jose Miguel de Barandiaran, and from witch trial records, above all the great Basque prosecutions of 1609 to 1614 in which the Spanish inquisitor Alonso de Salazar Frias investigated thousands of accusations and concluded that he had found no evidence of witchcraft at all. His report is one of the most important documents in the history of European scepticism and it happens to be about this region.
What survives of the older material centres on Mari, a female figure associated with mountains, caves and weather, with a companion figure Sugaar, and with a body of subordinate beings. The linguistic point is that none of this vocabulary has cognates anywhere. For Norse or Vedic religion, a divine name can be checked against related languages and its antiquity assessed. Here there is nothing to check against, so the age of any Basque religious term cannot be established by comparison, only by attestation. A language isolate is also an evidential isolate, and that is a real limit on what can be known. The divine names recorded in the Aquitanian inscriptions are the one exception, and they are a short list, cut in stone by people writing Latin. Everything else rests on folklore collected after nineteen centuries of Christianity, which is not nothing and is not a text either.
Sources Selected Writings of Jose Miguel de Barandiaran: Basque Prehistory and EthnographyJose Miguel de Barandiaran, ed. Jesus Altuna, trans. Frederick H. Fornoff, Linda White and Carys Evans-Corrales with The Witches' Advocate: Basque Witchcraft and the Spanish Inquisition, 1609-1614Gustav Henningsen