The Metamorphosis is a novel by Franz Kafka which has lots of religious symbolism.

For example, when an apple is thrown at Gregor and embedded in his skin. I heard that in religious symbolism, apple terms for sin. What, however, is the significance when the apple rots away? Does the sin rot away? That does not really make sense.

What I also don't get is what actually the sin is that
Gregor commits....
Can someone who read the novel help me
understand... please.....

I've read this, but it's been about 8 or 10 years, so some of the details escape me.

Try here: http://www.sparknotes.com/lit/metamorph/

Start out by reading about each of the main characters. Remember that Gregor had become the family's main income earner (breadwinner). What happens to the family when suddenly he stops going to work?

Be sure to read the summaries and the commentary.

Then don't skip "For Further Reading." This is a list of books, but by putting the titles of the ones that look promising into a good search engine, you'll find more information online that will help you form your own understanding of this complex work.

To understand the religious symbolism in Franz Kafka's novel The Metamorphosis, including the significance of the apple, it's helpful to look at the broader context and themes of the story.

The apple incident occurs when Gregor, the protagonist who has transformed into a giant insect, is pelted with apples by his father. While it's true that apples can symbolize sin in religious contexts, it's important to approach this particular symbolism within the context of the novel.

In The Metamorphosis, the apple represents not only sin but also a form of punishment and rejection. Gregor's transformation into an insect leads to his alienation from his family, who view him with fear and disgust. The act of throwing apples at him symbolizes the condemnation and rejection from his family, marking the beginning of their abandonment of him.

As for the rotting away of the apple, it does not necessarily mean that the sin itself is fading away. Instead, it could signify the deteriorating relationship between Gregor and his family. The rotting apple represents the decay and disintegration of their connection and their inability to confront or address the sin, guilt, or shame associated with their treatment of Gregor.

Regarding the sin Gregor commits, it's important to note that Kafka's novel doesn't explicitly state what specific sin Gregor might have committed. The focus of the story is more on the consequences of his transformation and the subsequent breakdown of his relationships. It may be interpreted that Gregor's sin is his inability to fulfill societal expectations and responsibilities, particularly as a breadwinner for his family.

In summary, interpreting the religious symbolism in The Metamorphosis necessitates examining the broader themes and context of the novel. The apple represents punishment, rejection, and the deteriorating relationship between Gregor and his family. The specific sin Gregor commits is left open to interpretation, but it could be seen as his failure to meet societal expectations.