After the Christianization of Scandinavia, were there tribes or ethnic groups that resisted it and maintained their Nordic beliefs? I mean the belief in Odin, Thor, etc, by which I leave the Sami / Lapp people out of the question. I appreciate that you leave bibliography if it exists.
This is a complicated question, and the answer is yes, and no.
Nominally, all were christian and all were under church law and church tithing. So there were no tribes or proto-states that resisted christianisation openly.
However, what people believed and did was an entirely different matter, and the old beliefs and traditions associated with them survived for a long time. Since Norse paganism never was a centralised religion with any kind of canon faith and story and varied from region to region, a lot of it morphed into local folklore. There are evidence of verses read over mares to increase their fertility that named Oden and Tor in the 17th century.
In many cases, it was not as much a question of faith as it was one about tradition versus new rituals and centralisation versus decentralisation. If one is to count the rune stones erected with christian crosses on them, a majority of Swedish Petty Kingdom Svealand was christian by 1000 CE, and a line of Kings of Svealand (and possibly also Östergötland and Västergötland) had been christian since Olof Skötkonung ~995- However, Inge the Older refused to conduct the traditional sacrifice to the Pagan Gods at Uppsala. The thing at Uppsala gave him an ultimatum - conduct the sacrifice or be deposed. His brother-in-law (according to the Hervarar Saga) Sven stepped forward and claimed to be willing to do the sacrifice and was promptly elected King in 1084.
Likewise, the people of Småland had supposedly reverted back to their old ways enough that Norwegian King Sigurd Jorsalafarar went on a combined looting expediction and crusade in 1123 to force them back under the church.
Likewise, the Second Swedish Crusade to Finland (the first one is heavily debated wether or not it is an actual historic event) was prompted by the Tavastlanders reverting back to their old faiths and a papal letter calling for their forced inclusion with the church in 1237. The timing of the crusade is debated, but most likely in the end of the 1230s.
According to oral traditions written down in the 17th century, at the Battle of Lena between Erik Knutsson (supported by Norwegians) and Sverker Karlsson (supported by Danes) 1208, Oden appeared in Norway, telling a smith there he rode to battle and then appeared at on the battlefield on a ridge behind the Danes, urging the tired Svealanders on, who hearted by this endorsement charged and drove the Danes into a marshy area, slaughtering most of them.
Regular people continued with rituals and sacrifices for a long time - especially in remote regions. There are evidence of Norese paganism having a small resugence in response to the plague in the 14 century and archeological finds with runic inscription on wood and bark pieces with magic verses or prayers to the Norse Gods have been found from the 13th century. Another example is how a statue of what was probably the fertility God Frej was taken to church, claimed to be a local saint and still carried out to the fields every Spring to bless their fertility.
The history of christianisation in Scandinavia is deeply intermingled with the creation of unitary Kingdoms, centralisation of power, the introduction of taxation and common law and church law and tithing. The lack of written sources from this period, the muddled situation with the sources that do exist (such as various monarch lists that conflict with each other) and the slowly morphing of the lower level of Norse paganism (with elves, dwarves, various other creatures) into folklore makes it hard to say when christianity beginds and Norse paganism ends. Those that elected Sven King in 1084 might not have been motivated by religious fervor but rather defending their autonomy against encroaching royal centralisation, using the old tradition as a symbol rather than a cause.
Some argues that the last vetiges of Norse paganism still exists in the form of placing milk and cookies or porridge for santa or the house elf, and my maternal grandmother always took care to warn any vättar (a folklore creature with its origins in Norse paganism) before throwing out hot water after doing the dishes and wahing herself at the cabin with no plumbing.
Likewise, looting and raiding continued to happen like the Småland Crusade by Sigurd Jorsalafarar 1123 and the Swedish Crusade to Finland in the late 1230s. How much these are expeditions to expand royal power, loot and pillage and to spread christianity can be debated.
So, yes and no.
In the 14th century, the upper class of the Numedal Valley in south eastern Norway were still building homes and other farm buildings with profane/pagan carvings. Germanic hero Gunnar aka Gunther from the Sigurd saga was a common theme. Traditional Biblical themes were absent during this time.
https://middelalderuka.no/index.php/en/middelalderdalen/kulturminner/22-gunnar-in-the-snake-pit
The Numedal Valley was an important route between Oslo and Bergen, and so priests and kings and pilgrims went right through there. And the carvings are out in the open for anyone to see, so it's not like they were hiding their paganism. How was this allowed?
Perhaps because "Sigurd dragon slayer has by many historians been seen as a parallel to the biblical dragon slayer St. George and the Christian Crusaders."
My ancestors are the Kravik family. Here's more on their buildings...
https://middelalderuka.no/index.php/en/middelalderdalen/private-bygninger/34-mellom-kravik
https://middelalderuka.no/index.php/en/middelalderdalen/private-bygninger/35-sore-kravik
"The second floor was the party loft guests were housed here when they were visiting the farm." Sure would have loved to been a guest back then!