On The Gods and Their Rites as Practiced Among the Germans, part I
Of the gods, they cherish Mercury the most, to whom on certain days they are accustomed by divine decree to offer sacrifices, human and otherwise. Hercules and Mars they lawfully appease with animals. And part of the Suebi make sacrifices to Isis: by what cause and source this foreign rite I have learned little, except that its own sigil, in style the form of a ship, proves it a religion brought hence from afar.
Deorum maxime Mercurium colunt, cui certis diebus humanis quo que hostiis litare fas habent. Herculem ac Martem concessis animalibus placant. pars Sueborum et Isidi sacrificat: unde cause et origo peregrino sacro parum comperi nisi quod signum ipsum in modum liburnae figuratum docet advectam religionem.
Tacitus, On the Origin and Disposition of the Germans, IX,1-2.
One aspect of ancient ethnographies is that they rarely fail to note occurrences of human sacrifice among their subjects. A second feature, at lest among Roman commentators, was the supposition that all the gods of foreign peoples were actually just different aspects of gods that the Romans already knew. But, from other details in Tacitus’ descriptions, it does not seem that Germanic cultic practice was much at all like that of the Romans. Rather, the Germans seem to have believed in worshiping their gods in the natural environment not in constructed temples or sanctuaries, sanctifying groves and copses of trees for ritual purpose. This aspect of German religion is one of the key points that Tacitus makes in his study, and appears in later sections of this same chapter.