"The history of the vikings was penned in the 1300s, after Scandinavia had become christian"

Adam of Bremen wrote about the norse gods and pre-Christian Scandinavian religious practices at around 1080 AD. The Norse sagas were written down between 1120 og 1350 AD, roughly speaking. They were orally transmitted before being written down. Not to mention the numerous references to the vikings in contemporary accounts, from Ahmad ibn Fadlān to frankish annals to english munks.

The old norse poetry have very ornate meter and syntax, meaning that the exact original wording of them would have been preserved for a long time orally.

Anyway. We know about norse mythology from three main sources (in norse literature): The Poetic Edda, the Prose Edda and some of the Icelandic sagas.

The Poetic Edda is the oldest, written down in 11th and 12th century, based on much older oral traditions, most of it in various forms of norse poetry. Since parts of these poems have been found elsewhere we know they date back far. The german Hildebrandslied from the 8th century have parts of the poetic Edda in it, for example. It was known in Iceland, since it was part of the culture the vikings brought with them there.

The Prose Edda, or younger Edda, was written by by Snorre Sturlason around 1220, and is a book dealing with norse poetry and norse mythology. It's thought that it was written as an attempt to preserve knowledge of the old myths and belief system, and to keep the old norse poetry from becoming forgotten.

It's not always clear, and this is much debated, if the Edda's, particularly the poetic one, is a genuine expression of the norse belief system composed by those who believed it, or if its the impressions of christian authors we see, their description of the the old belief system. The norse gods are often slightly comical in the way they are portrayed.

The Icelandic sagas takes place from around 930–1030, and were written down around 1190 to 1320 AD. Ynglinga saga, Njáls saga, Flateyjarbók all contain references to the norse gods and the shift from the old religion to christianity.

Also, belief in Odin persisted for a long time. During the battle of Lena, in 1208, Erik Knutsson with swedish men, and and a small number of norwegian men, fought against Sverker Karlsson. It was a fight for the swedish throne, with supporting factions on both sides. At the time it was said that Odin appeared riding on Sleipnir, aiding the winning side of the battle. So the victory was at the time attributed to Odin, which says something about how persistent the belief in him was.

So it's not technically wrong that a lot of stuff we know about the vikings were written down in the 13th century, but its really stretching the truth to the point where noses will grow and the seat of your pants will reach unfomfortable high temperatues to claim that it was all written down after everyone had become christians.

Nothing in that photo is false. EDIT: there were writings before 1300, but not before the Christianization of europe. As to the implied claim that Odin's hanging from a tree is derived from Christianity, that's virtually impossible to tell because (as the photo notes) the sources date from well after christianization of Scandinavia.

As to whether Scandinavian mythology was made up by Christians, I think that's a pretty absurd claim without some evidence. For every parallel story (which is not uncommon in unrelated cultures), there are many distinct Scandinavian stories.

Finally, runes attesting scenes and names from Scandinavian mythology predate christianization of Europe by several centuries. It would be difficult to explain these in a Christian context, to say the least.

Here is imgur of the probably false image that I did not realize was gonna link directly to a facebook group. Further apologies for my lack of knowledge in the facebooks :)