In The Old Man and the Sea, Ernest Hemingway uses a plain, straightfoward style of 3rd person narration that is common to most religious parables and allegories.
Hemingway uses many simple sentences, unadorned description, and a matter-of-fact dialogue to create believability. In short, Hemingway's style is without pathos (emotional appeal). Rather, the old man is based on logos (logic) and ethos (ethics).
The mood of the novella is based on religious allegory: it is a contemplation on man alone in nature suffering, much like Jesus' temptation in the desert or Jonah and the Whale.