500 years of Inquisition have left few remains to be picked up in the more illustrated 20th century by scholars, ironically some of them Christian priests.
Still we know something.
Basque religion is (as still shatters our souls in ways no other faith can) first and foremost a dualist-monotheistic religion focused on fertility. We know of only two gods: Mari and Sugaar (aka Maju), representing the female and male halves of such unique, yet dual cosmological principle.
Basque religion is an Earth-centered (ctonic) religion, where the sky is just a passage for the Goddess and God to travel and shepherd their herds of clouds and other metereological phenomena. They dwell in caves, in the most outstanding peaks sometimes. It's believed that whole ill-known realm lies underground. There, along with the dual Divinity, many magical criatures like lamiak (nymphs), iratxoak (imps) and jentilak (giants), along with the dead ancestors may live.
Izena duen guztia izan omen da. All that has a name may exist. Izena izana. The name [is] the existence.
Basque religion used to be celebrated specially on friday nights, the day of the akelarre (sabbat). It was a common practice still in the 15th century if we are to believe the inquisitors. Avellaneda was a skeptic: he did not believe in witches, so he went to the Pyrenees to find out what were all those rumors about. The locals welcomed him and made him participant of their celebration. Possibly under the effects of stramonium (or some other drug of similar type), Avellaneda flied and saw things that he would otherwise never have believed in. Back to Logroño he initiated one of the worst witch-hunts ever.
The case is that their "god" could not make such "miracles" and he was jealous.
It's said that certain witch apprentice had to recite the following enchantment to arrive to the akelarre: under the clouds, over the thorns. But he recited it wrongly: under the thorns, over the clouds, with disastrous effects. Same with Avellaneda.
But there were many other witch-hunts. And eventually the old religion went underground and vanished. Legend says that in other times lamiak were very common, being found almost in every stream, taking care of their long hair with their golden combs. Why did they disappear? The myth also has an answer: because of chapel building. In every holy spot the Christians built a chapel, one after the other, until all the mountains were Christianized, by grade or force.
And mythos in Greek means thread: Ariadne's thread.
You can follow the thread or get lost in the Labyrinth of Christian deceit.