It's really difficult to tell as most ideas about Nordic Spirituality and Mythology before Christianity was in Europe is mostly theorized reconstruction. Such as we don't know for sure that Tyr was the original chieftain god of the Teutonic pantheon, but comparative linguistics do lead us to believe that.
What we DO know about Ragnarok is that if it is pre-Christian, then there are a lot of coincidences. Christian Revelation talks about monsters and celestial entities bursting out of the sea and sky and anyone on Earth would be caught in the crossfire. There was the "beast of the sea" and "The beast of the earth" and also "the beast", which Christians, when talking with Nordic peoples, probably likened to Jormungand, Fenrir and Loki.
We also know that the Nordic people were very open to equating characters in lore. Anglo-Saxons, when explaining their practices to Romans, would say that they hold Mercury in the highest praise, and give him the fourth day of the week, and that they call him Wodan. It was also common to adopt practices, such as the days of the week mentioned before. That was originally a Roman practice, but the Nordic peoples adopted it. They even kept the Roman god's name for seventh day of the week, Saturday, which to them may have been a day to one of the Vanir gods, possibly Njord (as Saturn is a god of harvest and most Vanir gods have strong associations with harvest and agriculture).
Within Ragnarok Lore, we see a lot of Christian Mythological symbolism. There's the tree and the serpent, a man and a woman seeking refuge inside of it to repopulate the earth and also the return of the dying god, who so happens to be the son of the King of Heaven.
In my opinion, Christians probably used the Nordic peoples habit of equating characters and adopting practices to their advantage. Then once the Northern Europeans fully adopted the idea that the world will end, and believing that with Christian logic, then they could say that Ragnerok already happened, and our God is what you call Baldr who has returned.
Of course not everybody bought into this, but it was enough to get wealthy and powerful people to convert (with the assistance of possible underhanded economic exchanges or political agreements), as to make the conversion of the masses easier.