Everything that is happening in the Congo stems from the ivory trade. I believe this passage comes toward the end of the story in talking about Kurtz and his compound. Kurtz has fallen victim to the ivory trade. It has taken over his life and his greed for it has overpowered him so much so that he has people praying to it for him and with him. Kurtz was smart enough to be able to brainwash the natives into working for him, but he was unable to self-regulate and therefore, everything that is at his compound is geared toward the ivory. Marlow, on the other hand, didn't come to the Congo to get ivory, so he can keep his mindset based in reality. He was sent to save Kurtz and bring him back, but he finds that that is impossible.