I wonder how many people, like me, came to this question because they thought it was about a talking knife XD
It definitely needs to be a short sentence, because it would be written on the blade - which, for a knife, is generally short and thin. Moreover, general wear-and-tear (knives aren't exactly made to last well in a fight) would have a good chance of rubbing off the inscription.
So if, for example, it DID say, "Never give up without a fight", if damaged, it could end up reading something like, "Never <unidentifiable scrawl> fight", which is an odd thing to have on a blade.
(I should add that I say it'd be written on the blade, because knives don't have significant cross-guards or pommels, and it's a generally bad idea to write something on the grip - which, actually, I don't think a knife has much of either. So you'd be looking at the blade, right?)
I would suggest one word - something significant to the story and character. "Valor", "home", "love" and "blood" could all work in different contexts. What would work best for your plotline?