There it was cleaned and the bare skull was adorned with gold, as is their custom. It was used thereafter as a sacred vessel on special occasions and as a ritual drinking-cup by their priests and temple officials.

Nicander of Colophon [second century BC] (via Tertullian De anima 57.10):

It is often said because of visions in dreams that the dead truly live. The Nasamones receive special oracles by staying at the tombs of their parents, as Heraclides-or Nymphodorus or Herodotus-writes. The Celts also for the same reason spend the night near the tombs of their famous men, as Nicander affirms.

Posidonius [first century BC] (via Diodorus Siculus 5.28):

The teaching of Pythagoras prevails among the Gauls, that the souls of humans are immortal and that after a certain number of years they will live again, with the soul passing into another body. Because of this belief, some people at funerals will throw letters into the funeral pyre, so that those having passed on might read them.

(via Diodorus 5.31):

The Gauls have certain wise men and experts on the gods called Druids, as well as a highly respected class of seers. Through auguries and animal sacrifice these seers predict the future and no one dares to scoff at them. They have an especially odd and unbelievable method of divination for the most important matters. Having anointed a human victim, they stab him with a small knife in the area above the diaphragm. When the man has collapsed from the wound, they interpret the future by observing the nature of his fall, the convulsion of his limbs, and especially from the pattern of his spurting blood. In this type of divination, the seers place great trust in an ancient tradition of observation.

It is a custom among the Gauls to never perform a sacrifice without someone skilled in divine ways present. They say that those who know about the nature of the gods should offer thanks to them and make requests of them, as though these people spoke the same language as the gods. The Gauls, friends and foes alike, obey the rule of the priests and bards not only in time of peace but also during wars. It has often happened that just as two armies approached each other with swords drawn and spears ready, the Druids will step between the two sides and stop the fighting, as if they had cast a spell on wild beasts. Thus even among the wildest barbarians, anger yields to wisdom and the god of war respects the Muses.

(via Diodorus 5.32):

It is in keeping with their wildness and savage nature that they carry out particularly offensive religious practices. They will keep some criminal under guard for five years, then impale him on a pole in honor of their gods followed by burning him on an enormous pyre along with many other first fruits. They also use prisoners of war as sacrifices to the gods. Some of the Gauls will even sacrifice animals captured in war, either by slaying them, burning them, or by killing them with some other type of torture.

(via Strabo 4.4.4-5):

Generally speaking, there are three uniquely honored groups among the Gauls: Bards, Vates, and Druids. The Bards are singers and poets, while the Vates oversee sacred rites and examine natural phenomena. The Druids also study the ways of nature, but apply themselves to laws of morality as well. The Gauls consider the Druids the most just of people and so are entrusted with judging both public and private disputes. In the past, they even stopped battles which were about to begin and