Musing on the use of the word know in the movie and story got me thinking about what I find to be the most puzzling line of dialogue, to me. Ennis saying "What I don't know, all them things I don't know could get you killed if I should come to know them."
In response, Jack goes into a tirade, but an eloquent tirade, that ends with, "I wish I knew how to quit you." And he also says in that speech, "I hope you know that if you don't never know the rest." (Note: Jack's famous double negative crops up here.)
What was it that Ennis was so afraid to know that he had to threaten Jack about? Did all this have anything to do with the Tree of Knowledge that was the downfall of Adam and Eve?