In Commentarii de Bello Gallico, Julius Caesar wrote,1
[The Druids]2 likewise discuss and impart to the youth many things respecting the stars and their motion, respecting the extent of the world and of our earth, respecting the nature of things, respecting the power and the majesty of the immortal gods.
Multa praeterea de sideribus atque eorum motu, de mundi ac terrarum magnitudine, de rerum natura, de deorum immortalium vi ac potestate disputant et iuventuti tradunt.
The nation of all the Gauls is extremely devoted to superstitious rites,
Natio est omnis Gallorum admodum dedita religionibus,
They worship as their divinity, Mercury in particular, and have many images of him, and regard him as the inventor of all arts, they consider him the guide of their journeys and marches, and believe him to have great influence over the acquisition of gain and mercantile transactions. Next to him they worship Apollo, and Mars, and Jupiter, and Minerva; respecting these deities they have for the most part the same belief as other nations: that Apollo averts diseases, that Minerva imparts the invention of manufactures, that Jupiter possesses the sovereignty of the heavenly powers; that Mars presides over wars. To him, when they have determined to engage in battle, they commonly vow those things which they shall take in war. When they have conquered, they sacrifice whatever captured animals may have survived the conflict, and collect the other things into one place.
Deum maxime Mercurium colunt. Huius sunt plurima simulacra: hunc omnium inventorem artium ferunt, hunc viarum atque itinerum ducem, hunc ad quaestus pecuniae mercaturasque habere vim maximam arbitrantur. Post hunc Apollinem et Martem et Iovem et Minervam. De his eandem fere, quam reliquae gentes, habent opinionem: Apollinem morbos depellere, Minervam operum atque artificiorum initia tradere, Iovem imperium caelestium tenere, Martem bella regere. Huic, cum proelio dimicare constituerunt, ea quae bello ceperint plerumque devovent: cum superaverunt, animalia capta immolant reliquasque res in unum locum conferunt.
All the Gauls assert that they are descended from the god Dis, and say that this tradition has been handed down by the Druids. For that reason they compute the divisions of every season, not by the number of days, but of nights; they keep birthdays and the beginnings of months and years in such an order that the day follows the night.
Galli se omnes ab Dite patre prognatos praedicant idque ab druidibus proditum dicunt. Ob eam causam spatia omnis temporis non numero dierum sed noctium finiunt; dies natales et mensum et annorum initia sic observant ut noctem dies subsequatur.
1 Book 6, Ch. 14, §6
2 Book 6, Ch. 13, §1: “Throughout all Gaul there are two orders of those men who are of any rank and dignity... But of these two orders, one is that of the Druids, the other that of the knights.”
3 Book 6, Ch. 16, §1
4 Book 6, Ch. 17, §1–3
5 Book 6, Ch. 18, §1–2
Caesar, Julius. Commentaries on the Gallic and Civil Wars. Trans. Bohn, W. S.; McDevitte, W. A. 1st ed. New York: Harper, 1869.