In 98 CE, the Roman historian Publius Cornelius Tacitus wrote Germania, an ethnography of ancient Germanic civilization. It’s the oldest surviving written record of ancient Heathen customs.
According to Tacitus, the Germanic people worshipped the gods “Mercury” (Woden/Gwoden/Odin), “Hercules” (Thor), “Mars” (Tyr) and “Isis” (unknown goddess), and on Wednesdays they would sacrifice humans and animals to Odin. The Germanic people were superstitious and would consult a system of lot-casting routinely to predict omens. For temples, they preferred to worship outside in fields as opposed to in buildings, believing this was more appropriate than enshrining their gods. Unlike the Romans, they didn’t portray the gods in the likeness of human beings.
It’s unclear how objective Germania actually is as an ethnography. Tacitus never observed the Germanic people himself or even set foot in the region. Instead, he sourced information from other written works and perhaps interviews, which were second-hand at best. Additionally, Tacitus’s descriptions of the Germanic people are generally derogatory, portraying them as lazy, uncultured, drunken, and brutal.
Germania is nevertheless significant to Norse Heathenry because of its influence. As a source of inspiration during late-19th and early-20th century German Romanticism, it helped shape the core ideologies of both Odinism and Nazi beliefs. This is discussed in later entries.
You can read an english translation of Germania here.