Norway was converted well before you're setting your adventure - even with the pagan resurgence under the Jarls of Lade after the battle of Svolder in 1000 CE, Oláfr hinn helgi/Olaf II/Saint Olaf effectively stamped out all organized traces of paganism by the end of his reign in 1030. In 1154 the Church had established an Archdiocese at Nidaros, which was basically the declaration that Christianity was in total supremacy in Norwegian spiritual life.

If you're setting your story in the early part of the 13th century, you're going to be dealing with, effectively, a totally Christian population (or near enough to totally Christian as to not matter.)

If you're looking for academic sources, I'm going to recommend the following:

Anders Winroth, The conversion of Scandinavia: Vikings, merchants, and missionaries in the remaking of Northern Europe

Nora Berend, Christianization and the Rise of Christian Monarchy: Scandinavia, Central Europe and Rus' c. 900-1200

You might also want to read Elena Melnikova's "How Christian Were Viking Christians?" which is conveniently online in a pdf here.

That said, it's your fiction, so if you want to include Shadowy Pagan Cultists, then go for it.

Christianity’s spread in Norway was a process that took centuries. The first definitive signs of Christian influence in Norway are Celtic crosses plus some Christian burial procedures. Those things date back to 5th or 6th century CE. But it was still a long time before Christianity became a major influence. The first Christian king of Norway was Haakon the Good who reigned from 933 to 961. There were a number of Christians in Norway before this king was born – and there were still Old Norse believers after this king died.

In this time, Germanic tribes (like the Norwegians) made important decision in "Thing" which were gatherings with all free men. In a gathering at the island of Moster around 1020 CE, they decided that Christianity should be the religion of Norway. The great majority of the populace supported this decision, but there was still some contention. The struggle between Christianity and the old faith ended at the Battle of Stiklestad in 1030 CE. After that, the old faith was never again any significant “competitor” to the new religion.

In the early days of Christianity in Norway churches were privately owned. Rich farmers would build a church at their own expense and offer it as a service to the local community. Church organization grew gradually and by 1070 CE there were three bishoprics in Norway: Oslo, Selje (later Bergen) and Nidaros (known as Trondheim today). By this time Christianity quickly became more formalized

However, it is also a question of what was the Old Norse faith. The idea most people have when they think about this has very little to do with historic reality. The stories of Odin Thor, Loki, etc. mainly comes from a book by the Icelandic writer Snorri Sturlasson who lived 1178-1214 CE. His book Edda tells the story of the Norse gods. His book is not a documentary, but a mix of old Norse tradition, quite a lot from the Bible (!) – all combined with Snorre’s great talent to tell a good story. The Edda was written 200 years after the Old Norse religion disappeared, so it is not an eye-witness in any way

Therefore, Edda is not a good description of what Norwegians believed before the arrival of Christianity. The Norse religion did not have a systematic “theology” with clearly defined dogmas. Neither did they have a professional guild of religious leaders, although of course some people would have a leading position during religious rites. Norse religion was probably quite Animistic, where it is important to “control” nature through magic and rituals. It is also typical to celebrate seasonal challenges. Animistic beliefs can live pretty well together with more dogmatic religions (like Christianity). This is known as “syncretism”. That means that some of the old beliefs and practices could continue for a very long time after people adopted Christianity. People would believe in Christ and the Bible, but at the same time they would believe in powers of nature and even spiritual forces and beings unknown to “standard Christianity”. Even today, the Norwegian word for Christmas (‘jul’) comes from the old Norse celebration of winter solstice - and not from Christ as in English. Certain Christmas traditions in Northern Europe have its roots in pre-Christian religion. From this, you can argue that 0.1 % of Old Norse faith is still around

I have studied all of this in Norwegian, so I do not have many helpful links in English, but maybe you can research some of the names and event mentioned above. For a good picture of life in rural medieval Norway (13th century) you may want to read the Nobel Prize winning trilogy Kristin Lavransdatter by Sigrid Undset. These books touches quite a lot on religious ideas and practices in that time

Thanks for the answers! One more thing popped up.

I've read (at wikipedia btw, which might not be correct, but I use it to try and get a general feel for things I'm not educated in) that norwegian law from 1160 has regulations concerning domestic slaves, and laws from 1270's mention former slaves.

Does this mean that some form of thraldom was still practiced around 1220, or is it simply that certain articles of the law survived long after they fell out of use?

Thanks for the answers! One more thing popped up.

I've read (at wikipedia btw, which might not be correct, but I use it to try and get a general feel for things I'm not educated in) that norwegian law from 1160 has regulations concerning domestic slaves, and laws from 1270's mention former slaves.

Does this mean that some form of thraldom was still practiced around 1220, or is it simply that certain articles of the law survived long after they fell out of use?