Answer:
Marlow talks about death and the afterlife (if any).
Explanation:
The moment Marlow talks about is the time just before Kurtz' death. This becomes evident after mentioning "living his life again", which is a common belief about what happens before you die.
"Complete knowledge" refers to finally knowing what there is after death. For a long time people have wondered what lies beyond our lifetime. Kurtz will now have the opportunity to possess that knowledge.
