It all started with their kings being “converted” for economic benefit and trickled down. There were probably huge periods (not very well documented given the times) where there was an “official” religion while the general population went about their business ignoring it.
But there were also a lot of cases in Scandinavia in particular where there was a “melding” of some kind between two religions, before Christianity became the majority religion. Thor’s hammers that were found looking more and more like Christian crosses, that kind of thing. Pretty sure Ireland was similar. Like Jesus was added to the pantheon as a compromise at first as a foothold etc.
But in times where there wasn’t a huge culture of written records (and oral history is seriously overrated as accurate when being handed down) it just makes sense that after a few generations people honestly didn’t really remember their great-grandparents religions etc.
Sometimes it started out for pragmatic and political reasons. Harald Bluetooth of Denmark changed the State religion to avoid getting into a holy war with the newly formed and enormous holy roman Empire.
As for the everyday person the old ways just gradually faded out in favor of the wealthy and influencial church and the benefits monks brought in terms of foreign medicin, learning and herbs.
Active conversion efforts, better trade opportunities, violent conversion efforts, the allure of belonging to a secretive or selective group, for the Roman empire enveloping religious traditions of conquered peoples into your own myths was a deliberate piece of their statecraft.
Lots of individual and immediately understandable reasons we don’t really have to guess as, they are happening today. People change their religious affiliations today all the time. You can ask them and get answers. People still claim to speak with and/or for some version of the divine and even today you will see such traditions carried on by doomsday prophets on the sidewalk and twats behind the pulpits of mega churches.
Scientology exists, Thousands of years of Abrahamic Hindu and Buddhist proliferation and a charismatic nutter can still talk his way into enough power to become his own Jesus but with sci-fi and hookers. There are plenty of cults that manage to draw in people and grow.
If a powerful enough leader gets sucked into a cult, or maybe they need their cults support to navigate some tricky political drama then you can get a real power behind the spread of a religion. As christians are to Justinian so SeaOrg is to Tom Cruise.
christianity was introduced to scandinavia by force. 400 years of systematic mass murder and indoctrination was used to remove the Norse gods and make christianity mandatory. Not following christian customs, or the worship of anything other than the christian gods were illegal by law and punishable by death. Draconian laws making christian customs mandatory was in effect all the way to the 20th century, it was not until 2015 that the blasphemy law was finally removed in Norway. Mandatory christianity was constantly publicly challenged during the 20th century in Norway, and from the 70s and onwards most Norwegians started to agree that christianity is fucking bullshit, and Norway is a atheist/non religious majority country today. Fewer and fewer Norwegians believe in christianity, to the point where islam is predicted to be a bigger religion than christianity in around 50 years. Not because people convert to islam, but because people turn away from christianity. There still is a lot of culturally christian people in Norway, but fewer and fewer believe in gods.
For the particular case of [Iceland](https://en.m.wikipedia.org/wiki/History_of_Christianity_in_Iceland), the whole thing was decided in on legal grounds. Parliament (called “Althing” in Iceland), at their meeting in either the year 999 or 1000, appointed the pagan law speaker (sometimes translates a “lawyer”) [Thorgeir Thorkelsson](https://en.m.wikipedia.org/wiki/Thorgeir_Ljosvetningagodi) to settle the matter. Facing unrest between pagans and Christians, Thorgeir (a pagan) lay under a blanket for a day before deciding in favor of Christianity. Pagan sacrifice was allowed to continue in secret, provided that no witnesses could be produced to it.
The wiki page quotes a historian as saying this account is unbelievable, but the only one they have. Certainly a very interesting chapter in history.
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