I just want to add to what @cassandresaid about sin, because it is an important part of the novel and there is a lot think about. I think that there are 2 ways to look at this - firstly from a historical perpective and secondly as part of the literary tradition. (I’ll caveat this by saying I’m not a historian & I don’t know much about Norwegian literature).
From a historical point of view, medieval rules about chastity were strict, and the consequences for women for flouting them could be severe - although not always. A good example is Queen Isabella of England who got away with a presumed affair (until she got into trouble politically) because her husband (Edward II) was so inept - in contrast 2 of her sisters-in-law (married to French princes) were imprisoned for adultery because Isabella’s father was a much stronger king. Men tended to escape this censure, although not always (Pierre Abelard was castrated for his affair with Héloïse) and they could be resented for this (I’m think I remember that this was one of the complaints against King John). The point about confession and absolution is a really good one - but while I would agree with expecting someone to move onwards after that, I can’t help wondering if this might be depend on what else happens in your life. Admittedly, that’s due to thinking about Henry VIII continuing to think that his first marriage was sinful because he has no sons despite what the church told him, and he probably isn’t a strong argument for anything.
One of the things I really like about this novel is that as well as looking at what the author and the characters say, you also have to bear in mind what the characters do, and it is really striking that if the main women characters in the novel, few actually live up to medieval moral standards. The only one does is Ulvhild, who is (1) very young and (2) very unwell - Kristin, Ragnfrid, Fru Aashild and Simon’s sister all fall short. Kristin’s friend at the convent would have probably liked the opportunity to sin and Simon’s mother is possibly cross that Kristin has behaved too properly towards Simon. My reading of that is that it is quite a damming critique of medieval morals - they just aren’t realistic for most people.
Some men do fare better on this, but it is striking that of the “good” men, none are celibate. Lavrans and Simon both marry, and Gunnulf pointedly doesn’t engage with the conversation when Erlend asks about his celibacy. Sigrid Undset also makes it clear that they have all paid a high price for conforming to societal standards (Lavrans and Simon would prefer to be married to someone else and Gunnulf presumably would rather not be in the Church). Again it shows that the system doesn’t work for men either - you either pay the price for conforming, or (like Erlend and Bjorn) you lose the respect of your peers.
I also think that it isn’t the sin that is such a problem for Kristin and Ragnfrid - it’s more that their behaviour has been so secretive and deceptive. Kristin herself shows that public opinion isn’t always as strict as moral standards when she thinks that people would be sympathetic if she was pregnant if her father had kept her and Erlend apart because it is romantic, but they will think it funny that she and Erlend could not wait until they were married. Simon’s sister’s situation is very sad when her fiancé dies unexpectedly, but her experience is very different to that of Kristin. Simon and his wife look after his sister while the baby is born, the baby’s father’s family adopt the baby and Simon’s sister then returns to her family. It is a very sad situation, but the impression is of family rallying around to support someone who had been very unlucky.
Ragnfrid, I think is haunted by the fact that she deceived Lavrans when they were married and she must have been under a lot of pressure as it is implied that Fru Aashild knew about this so could betray her - then this was compounded by her later betrayal and being unsure of who the father of her first child was. The deaths of her children and Ulvhild’s injury, I think really confirmed her belief that she was being punished for her behaviour. Ulvhild’s situation is particularly sad as I think there is a hint in the book that Lavrans and Ragnfrid were getting on much better after Ulvhild’s birth because Ragnfrid was much happier (and prettier). This just makes her situation sadder as she is trapped by her own guilt.
For Kristin, I think that she was betraying her commitment to Simon and deceiving her parents and the abbess - and that it is hard to sustain the image of a romantic love when her behaviour has been so squalid at times (eg meeting Erlend in a brothel). I think it is this deceit that is so hard for both Ragnfrid and Kristin to bear and this does distinguish them from Simon’s sister. I’m also wondering how Kristin’s reckoning with her own behaviour while on pilgrimage will affect her relationship with Erlend, who hasn’t had this experience (because he went hunting).
I’ll write a bit more later re my thoughts on the litarary tradition - & thanks again to @cassandrefor making me think so much about this aspect of the book.