Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
No man is an Iland, intire of it selfe; every man is a peece of the Continent, a part of the maine; if a Clod bee washed away by the Sea, Europe is the lesse, as well as if a Promontorie were, as well as if a Mannor of thy friends or of thine owne were; any mans death diminishes me, because I am involved in Mankinde; And therefore never send to know for whom the bell tolls; It tolls for thee.
–John Donne
Hemingway didn't pick John Donne for the epigraph just to get a weighty sounding title out of it, nor because he thought the spelling was amusing (though both may have been factors). More important is that the epigraph makes a philosophical point that is central to the book. Simply put, the idea is that no individual person is really all "on their own," but always bound up with other people. In some way, every other person is a part of your own self, of who you really are – so when another person is lost, we lose something too. Hence the idea that whenever the bell tolls, it tolls for you – whenever somebody dies, it's as if a part of you dies (there's death again).
Pretty deep – Donne isn't called a "metaphysical poet" for nothing (Check out Shmoop Poetry to see what we mean). And if you thought that Hemingway was just a tough guy with tough little pill-like sentences, think again. There's a lot more to the book than that, such as the question of whether human beings really do live in communion with each other.
What do you think of that idea? Perhaps it makes sense when applied to one's lover, or friends, or family – all of those relationships play a very significant part in Robert Jordan's sense of self. But does it really extend to everyone else, to all of "Mankinde," including the very enemies who are fighting against you in a war and killing your own loved ones?
Donne's vision of human communion sounds nice and pretty in the abstract (that is, as an idea removed from reality), but in the face of reality, and especially in the face of war, it runs up against some major issues. You might not want to simply assume that Hemingway agrees with his epigraph. Perhaps the book's title is asking whether Donne's idea is actually true. But don't assume he just rejects it either. It might be a bit of both.