There is no one single "moral" in the book "The Scarlet Letter," though there are several themes that include looking at morality through sin and knowledge, the human condition, identity, society and the nature of evil itself. There are also several motifs throughout the book, including civilization versus the wilderness, evocative names, night versus day and of course, the symbolism of the scarlet letter.
The theme of sin and knowledge is one of the most talked about themes in "The Scarlet Letter" because both are linked in the Judeo-Christian tradition. In the Bible, Adam and Eve are forced to leave the Garden of Eden after they eat from the tree of knowledge. The knowledge is considered bad. However, the book shows that sin can lead to the acquisition of sympathy and to personal growth, as well as a better understanding of other people that cannot be found in a state of purity.
The scarlet letter is one of the most well-known symbols in the book, as it is also the title of the book. The scarlet letter is supposed to be a symbol of shame, but it becomes a powerful identity symbol to Hester, and instead of marking him as an "adulterer," it starts to stand for "able." It is used to showcase a theme in the story that the community's system of judgment and punishment is meaningless and that while the child in the story is sent directly from God, this letter is something created by humans. It also questions the ability of society to use various symbols.