Norse mythology is known chiefly from thirteenth-century Iceland, roughly two hundred years after the Icelanders adopted Christianity, and the two principal sources were made by Christians for purposes of their own.
Snorri Sturluson wrote the Prose Edda as a handbook for poets. Skaldic verse depends on allusive compounds drawn from myth, and a poet who no longer knew the stories could not read or compose it, so Snorri set the myths out systematically to keep the craft alive. He arranges, explains and occasionally rationalises, and he opens by framing the gods in terms a Christian reader could accept. The Poetic Edda, a manuscript collection of older verse, is less mediated but still a selection made at the same period.
The contemporary evidence is of a different kind and does not tell stories. Runic inscriptions are short and mostly commemorative. Grave goods, ship burials and picture stones show practice rather than doctrine. Place names record which gods were worshipped where. Outsiders wrote descriptions, Tacitus in the first century and Adam of Bremen in the eleventh, and both had agendas.
What follows is that the tidy pantheon familiar from popular retellings is partly Snorri's organisation of material that was never that tidy. Regional variation was substantial, the archaeological record does not always match the literary one, and much modern reconstruction of the religion is nineteenth century or later.