I've created this thread to settle a small debate between two friends of mine regarding how much Christian beliefs and ideas from their neighbours influenced the mythology of Norse paganism.
One of them is adamant that Ragnarok is a creation of Christian missionaries or later Christian revisionism of the old myths. He is also of the opinion that the role of Loki was "rewritten" from a neutral trickster god to the a more Satan-like position by this Christianisation of Viking culture.
The other person in the debate takes the opinion that this apparent change in the nature of various entities in the Norse myths are the result of natural change over time. I'm inclined to agree with this view, although I would think that Christianity did have an influence but more due to cultural osmosis rather than intentional revisionism.
Basically, what we want to know is how Christianity may have influenced the mythology and religion of the Viking peoples.
There's speculation on that, but I don't see how one could say anything for certain. The most common opinion now is that the Völuspá (the main source on Ragnarök) was written around 900-1100, so during the period at which Christianity caught hold. Parallels to Jesus have also been drawn to Snorre's version of the Baldr myth. But there's no obvious Christian influence on either (or there wouldn't be much discussion). At the very least, key elements of the Christian apocalypse, such as resurrection and judgement, are missing from Ragnarök.
By contrast, the 12th century Christian apocryphal text Nidrstigningar Saga (a Scandinavian version of the Gospel of Nicodemus) is so explicit in its references to Norse mythology that it's more or less syncretistic. E.g. Satan is described as a giant (jøtunn), he is the king of trolls, and the Midgard Serpent is in league with him.
If you compare stories that have multiple attestations, they can differ hugely - Saxo Grammaticus' account of Baldr and his death is very different from Snorre's. It could be due an influence of Christianity, or that the versions stemmed from different times. But they also stemmed from different locales. I emphasize that there was no unified 'viking' people, nor culture, nor religion. The archaeologist Fredrik Svanberg has identified 5 different burial customs during the Viking Age in Scania alone. There were only similar customs across large geographic areas among the wealthiest class of people. The majority of ordinary Scandinavians of the Viking age may have had a quite heterogeneous set of beliefs.