Symbolism, Imagery, Allegory
In the "Character Analysis" for both Marlow and Kurtz we talked about how Conrad compares both men to gods. But it's not that simple. (We hope that doesn't come as a surprise.)
So, let's try to unpack this. Marlow is like a Buddha who, last time we checked, was an enlightening teacher-figure. Kurtz, on the other hand, is described as a lightning and fire Jupiter figure. Jupiter was a little more prone to the negative human emotions of jealousy, vengeance, and ambition. So right away, the god imagery allows us to differentiate between our two big characters.
But it doesn't end there. Interestingly, Marlow calls the white men on the ship "pilgrims." Like the Puritans at Plymouth Rock, remember that "pilgrim" is a word for people embarking on a religious journey for spiritual reasons. This may just be ironic, since altruistic enlightenment was one of the supposed motives for England's imperialistic forays into Africa. Or the label of "pilgrim" may just infuse the tale with a spiritual undercurrent, making Marlow's discussions of darkness and light sound religious. Take your pick. Or come up with a new idea!