A factor of the novel, is to include that the characters do not know what they should do about the will of God, as if God did not inform things correctly. We see at the beginning of Genesis that God informed all things before they even happened. So the thematic construction should be done that God would inform what he wants, and not the idea of a God who leaves the person without knowing the correct way to do it.
In the second chapter of the novel, we see the scene of Cain bearing fruit, and Abel the lamb. In the plot of the novel's plot, the idea generated is that Cain's offer was not accepted, because he chose bad fruits. And I accept Abel's offer, for choosing the best of the sheep. So it was defined in the plot, that the problem was in the choice, and not in the object offered.
But was the problem really that Cain did not offer the best fruit, but that it was wrong to offer fruit? If we read in Hebrews 9:22, we have that without bloodshed not remission, remission, means forgiveness, indulgence, or mercy. But such bloodshed is linked to the verse of John 1:29, which says that Jesus is the lamb of God who takes away the sin of the world. Was not the offer determined for Cain, also a lamb? If we read John 5:39, we have that the scriptures testify of Jesus. So, wouldn't it be the act of offering a lamb as the definition that Jesus is the lamb that takes away the sin of the world?
To determine this, we must see if we have the period when the Bible quotes to offer a holocaust. If we read Genesis 8:20 we have that Noah offered animals for a burnt offering on the altar after he was saved from the flood. But there is no text saying that it was to offer a holocaust before, which defines that it was already a known fact before the flood. And that connects with the act of Jesus being the lamb of God, even if such people do not understand it. We can be even more certain of this by looking at the verse in Genesis 22: 7 where Isaac asks Abraham, where is the lamb for a burnt offering. Which clearly determines that such people in Genesis, knew of the concept of lamb for a burnt offering, representing, the lamb of God that takes away the sin of the world.
So the act of defining that Cain's mistake was choosing bad fruits, and not representing the lamb of God who takes away the sin of the world, to make the connection that those offerings that God accepts, is Jesus, and not what we have to offer. It generates a doctrinal problem, or a doctrinal failure at that point.
The real fact would be that both Cain and Abel should offer a representation of Christ, an animal that represented the death of Christ, and that representation would determine that nothing that man has, he can give as a remission of sins, only Jesus can redeem sins. Clearly defining that only Jesus can be the bridge between God and man. If we read Acts 4:12 we have that there is none other than Jesus Christ. And therefore, we have that Cain's mistake was not to obey God, and to offer what God does not determine, because what determines is the lamb of God that takes away the sin of the world.