A problem of sources
Most of what we know about the gods of pre-Christian northern Europe comes from writers who were Christians. Before conversion, the peoples of Scandinavia, the British Isles and the Germanic lands had their own religions, but they mostly did not leave long written accounts. When writing in their languages began, it was in the hands of Christian clergy and scholars. So the stories of the gods reach us through a filter. That does not make them worthless, but it means every detail needs asking: who wrote this, when, and why?
The Norse sources
The richest source for Norse mythology is Icelandic. Two works matter most. The Poetic Edda, also called the Elder Edda, is a collection of poems about gods and heroes that survives chiefly in a thirteenth-century manuscript, though the poems themselves are thought to be older and their dates are debated. The Prose Edda was written in the early thirteenth century by Snorri Sturluson, an Icelandic chieftain and scholar. It was designed in part as a handbook for poets, who needed to understand the old myths because skaldic verse is full of allusions to them.
In these works we meet Odin, the one-eyed god of wisdom and war, Thor with his hammer, Freyr and Freyja, the trickster Loki, and the story of the end of the world at Ragnarok. Snorri presents the gods in a way that a Christian could accept: he treats them in part as stories, sometimes suggesting that they were based on ancient people who came to be worshipped. This approach, known as euhemerism, was a common way for medieval Christians to explain pagan gods without endorsing them.
Iceland adopted Christianity around the year 1000 through a decision at the Icelandic Althing, and the myths were then written down roughly two centuries or more later. That gap is a reason for caution. Some details, such as the full cosmology of nine worlds, may owe something to later writers, and some may reflect Christian ideas. Scholars debate this and do not agree.
Other Scandinavian sources add pieces. Saxo Grammaticus, a Danish writer around 1200, retold ancient stories in Latin and turned gods into heroes and kings. Adam of Bremen, an eleventh-century German churchman, describes a major pagan temple at Uppsala in Sweden, but his account comes from reports and is hostile in tone. Runic inscriptions and carved stones show gods and symbols without telling the full stories. Thor’s hammer pendants from the Viking Age suggest personal devotion, though they do not explain what people believed.
Celtic gods and Irish tales
Celtic evidence is different. In the Roman period, inscriptions in Gaul and Britain name many local gods, often equating them with Roman ones. The Celtic stories of the Middle Ages survive mainly in Ireland and Wales. Irish monks wrote tales of the Tuatha De Danann, a race of supernatural beings who were defeated and withdrew into mounds and the Otherworld. In some versions they are treated as ancient people with magical gifts, not as gods in the strict sense. Whether the scribes consciously reduced the gods to history is a question that scholars discuss, and the answer probably varies from tale to tale.
In Wales the Mabinogion preserves stories with figures that look like divine beings, such as Rhiannon and Lleu, but the tales have been shaped as medieval narratives. Across the Celtic kingdoms more widely, we see old stories surviving as legend, not as living religion.
Anglo-Saxon and continental traces
For the early English, evidence is thin. The Venerable Bede, writing in the eighth century, mentions that the names of some months came from pagan festivals, including a goddess named Eostre, though his remark is the only early evidence for her and its meaning is debated. The days of the week preserve traces of the old gods: Wednesday echoes Woden, Thursday echoes Thunor, and Friday is linked with a goddess. Several early English royal genealogies trace the royal line back to Woden. Christian scribes who copied them apparently saw no harm in it, treating the god as a distant ancestor.
On the Slavic and Baltic side, writing is later still. The Russian Primary Chronicle describes idols set up in Kiev before the conversion of Rus, including one of Perun, but it was composed by Christian monks long afterward. In the same way, Lithuania remained pagan well into the later fourteenth century, and what we know of its religion comes largely from hostile outsiders.
Why the stories survived
There were several reasons for survival. For Icelanders, the old poetry was a source of national pride and poetic craft. For Irish scribes, native learning was valued, and their monasteries were part of a literate elite that cared about family and legend. Elsewhere, tales lingered in folk memory, in place names, in customs and in stories that hid a god behind a saint or a hero.
Christian authors also had conflicting motives. Some condemned the gods as demons. Others treated them as misguided ancestors, or as poetic ornament. Many simply recorded what they found interesting, trusting that their readers would not mistake it for belief.
How to read them
A cautious reader holds two ideas together. The stories are real medieval texts, and their authors knew things about older traditions. But they are not neutral records of pagan belief, and we cannot assume that a tale written in the thirteenth century shows what people thought in the eighth. When a book states firmly how the pagan Norse or Celts “believed,” it is usually filling gaps that the sources leave open.