On The Gods and Their Rites as Practiced Among the Germans, part II.
Concerning the rest, they would never think to confine the gods by temple walls, nor to counterfeit into any limited human form the magnitude of the celestials: sacred groves and woods they consecrate, and designate as godly that mystery which they view with solitary reverence.
Ceterum nec cohibere parietibus deos neque in ullam humani oris speciem adsimulare ex magnitudine caelestium arbitrantur: lucos ac nemora consecrant deorumque nominibus appellant secretum illud, quod sola reverentia vident.
Tacitus, On the Origin and Disposition of the Germans, IX,3.
Despite Tacitus’s ethnocentrism and belief in the superiority of Roman culture, he also begrudgingly admires the German subjects of his ethnography as pure and untainted by the corruption of luxury that has, in his mind, weakened the empire. One purpose of his book may have been as a challenge and exhortation to the Roman elite to find a way to return to the rough virtue and stoic strength of the old Republican mores, especially to the new emperor Trajan (r. 98–17) whom he served as a former consul, a senator, and possibly as a proconsular provincial governor.