What do the opening descriptions of Marlow compare him to, an angel, Jesus, Buddha, or Neptune?
The answer to this question can be found in one of Conrad's descriptions early on in the novel:
...[Marlow] began again, lifting one arm from the elbow, the palm of the hand outwards, so that, with his legs folded before him, he had the pose of a Buddha preaching in European clothes and without a lotus flower... (10)
As you can see, Conrad very obviously compares Marlow to the Buddha. This comparison is significant because it sets him apart from Christianity, which, for most of the English sailors listening to his tale, would be the most familiar religious culture of the day. In this way, Conrad very subtly identifies Marlow as an outsider. Even more importantly, he identifies him as an "enlightened" outsider, one who sees into the true nature of things, as the Buddha is traditionally conceived to be an enlightened and wise being who studies the complex threads of existence. Thus, through this small and seemingly simple description, the astoundingly dense nature of Conrad's writing becomes apparent, as he's able to convey immense amounts of meaning within the span of a single sentence.