Vinland Saga protagonist Thorfinn has lived through many traumatic events since his early childhood. He witnessed his father get killed right in front of him when he was just six years old, subsequently spending the rest of his childhood and teen years either in war or living as a slave. Vinland Saga manga readers can understand his current stance on violence and war because of this. However, being understandable doesn't mean he's right.
Eyvar, a farmer who joins forces with Thorfinn to colonize Vinland, doesn't know what kind of life Thorfinn lived before he returned home. Unaware that Thorfinn fought in wars for years and was an incredibly skilled warrior, Eyvar firmly believes Thorfinn's no-weapons stance is being naive. It's understandable why Eyvar feels how he does -- and he's at least partially correct.
Eyvar logically claims Thorfinn's rule against swords and other weapons in Vinland is nonsensical and dangerous, insisting the time of their necessity will come. It's hard to argue against Eyvar on this, considering the time period Vinland Saga's characters live in. Most men in these times resort to violence for the simplest of matters, and many kill for entertainment.
Thorfinn disagrees that swords are necessary and hopes to completely avoid violence, saying there can't be true peace if swords are involved. Eyvar says having swords is important because that's how peace is kept. Thorfinn doesn't have a real answer when he's asked how he plans to handle the situation if they get into conflict with people in Vinland.
Eyvar assumes the reason Thorfinn doesn't want weapons in Vinland, and doesn't like war in general, is because he doesn't understand how the world works. This assumption is obviously wrong. The irony of the situation is that it's precisely because Thorfinn knows how the world works that he has banned weapons in Vinland and hates war. He made this policy as a way to reject the way the world works because he despises it.
However, Eyvar is correct that Thorfinn is being naive by assuming swords won't be necessary. He has already admitted there are aspects of the whole Vinland situation he didn't consider, including what he plans to do if a group of violent people wielding weapons shows up and refuses to talk anything through. Thorfinn is still a skilled fighter and has proven he can take on multiple opponents at once if needed, but that can only go so far.
If an entire army shows up on Vinland and decides to attack Thorfinn and the other settlers, they'll be almost completely helpless against weapons. He alone can't fight an army, and Eyvar's lone sword isn't enough. Thorfinn will undoubtedly blame himself if at least one person on Vinland ends up getting killed and a sword could've prevented it -- and Eyvar will surely blame him as well.
Thorfinn is in an interesting position because if Eyvar is proven correct about swords being necessary, his character could go one of two ways as Vinland Saga continues. He could become even more strict about having no weapons in Vinland, or he could be forced to rethink the rule and get rid of it. Either way, Thorfinn will have to confront the fact that he's being irrational due to his trauma sooner or later.