The religious imagery in this novel is a motif. A motif is a literary device (such as imagery, symbolism, etc.) that occurs throughout the novel and helps readers understand the theme. You can find many metaphors for Christianity in the novel. Santiago is a type of "Christ" figure. His hands are wounded like Christ's; he carries his "mast" up the hill to his hut like Christ carried his cross; he is a fisher of fish whereas Christ was a fisher of men; he has a disciple, Manolin, etc. Some of the themes of this novel center around struggle, defeat and death. These are also themes in life. In the Christian world view, life on this earth is a constant struggle and in the end, there is death, but death is not a defeat for the Christian. Nor is death a defeat for the Marlin. He is such a magnificent and worthy creature that he transcends death because the memory of Santiago's struggle against him lives on. Santiago worries that the people that will eat the marlin's meat are not worthy, and then the sharks eat the meat before Santiago can bring the fish back to shore. In the Christian world view, mankind is not worthy of the sacrifice that Christ made because men are all sinners, but the Bible says that this is the very reason Christ came - to save sinners.
Not everyone interprets the theme from a Christian world view, but this is my opinion. I believe the Christian imagery is too significant to not think it relates to the theme. Read the analysis here on enotes and see what YOU think!