This chapter is pretty creative. I feel this is a satirical take on Christianity. The author is trying to say that Jesus' sacrifice on the cross should not be seen as a Get Out of Jail Free card.
There are a lot of cases of Christians who did many sinful things, and they believe that Jesus will absolve their sins as long as they believe in Jesus. They usually pile their sins to a mountain-size level before truly feeling remorseful. This chapter tried to criticize these kinds of Christians. The professor exemplified this type of Christian (and probably the sailor as well because he was depicted as a possible perpetrator of incestual sexual activity). When he knew about the existence of the "Jesus-like" person, he decided to do as many sinful things as possible as if tomorrow would never come. Finally, his sinfulness level became tremendously high, and at this point, he became motivated to seek the cross in the middle of the ocean to absolve his sins.
However, who knows what would happen if a random mountaineer suddenly found the cross on the other side of the earth? This "accident" negated the "Jesus-like" person's sacrifice on the cross and caused the professor and his colleagues to return to the state of mortal sin.
As a religious Christian, this chapter reminded me of the importance of not only believing in the Saviour but also doing good deeds.