Marlow asks the question "Did [Kurtz] live his life again in every detail of desire, temptation, and surrender during that supreme moment of complete knowledge?" To what moment is Marlow referring? To what complete knowledge?

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.