What is the atmosphere of a significant event in the novel The Old Man and the Sea? How is it created, and what purpose is it serving to advance plot, character, or theme?

The Old Man and the Sea is American writer Ernest Hemingway's 1951 novel about a Cuban fisherman attempting to land an enormous marlin after a several months drought of successful angling.
The events on the third day of the protagonist, Santiago's, quest are significant as they solidify a relationship that has been alluded to earlier, that is, the old man's communion with the marlin; it also demonstrates the man's ultimate redemption, despite his eventual loss of the fish to the sea.
The atmosphere at this point in the story is intense, and Hemingway dispenses with perfunctory descriptions of setting to focus on both the old man's outward struggle with the marlin, and his inward struggle with fate. In point of the latter, he remarks :

Fish, you are going to have to die anyway. Do you have to kill me too?

In fact, just a few passages later, the totality of the struggle with the marlin is underscored by Santiago's resigned and indifferent declaration to the fish, "come on and kill me. I do not care who kills who."
The atmosphere that Hemingway crafts at this point is also a militant type of sentimentalism. Santiago is cast as a soldier in battle doing what he knows to be his duty but, ultimately, not taking joy in the ritual. This helps to even his character. While he has been portrayed as one who takes pride in his abilities as an angler, his communion with the marlin in the final moments of its life allows the reader to also see his humility. Indeed, in describing Santiago's exhaustion, Hemingway also remarks on the old man's "long gone pride."
Santiago's calling in life was as a fisherman and, at this moment, we see that his destruction of the marlin redeemed his life's destiny. "You loved him when he was alive and you loved him after" Santiago tells himself. "If you love him, it is not a sin to kill him. Or is it more?"