Unlike many novels, Hawthorne tells us main theme or moral in the last chapters. Hawthorne writes, "Among many morals which press upon us from the poor minister's miserable experience, we put only this into a sentence:—“Be true! Be true! Be true! Show freely to the world, if not your worst, yet some trait whereby the worst may be inferred!” In other words, do not be a hypocrite. Do not put on a false front to the world to make it seem like you have no faults. It is OK to let others know you are not perfect. This idea is exemplified in the life of Dimmesdale and Hester. Dimmesdale tried to hide his sin and guilt from the world. As a result, he was eaten alive by remorse and guilt, his heart literally weakened, and he died. Hester, on the other hand, never hid her sin and learned to rise above it by becoming humble and doing good deeds. She becomes stronger and more respected in the end that Dimmesdale, who loses all credibility with the people of Boston at the end of the novel.
The main theme of the scarlet letter is to be true. Hawthorne clearly says to "be true, be true, be true" at the end of the novel. Hester and Dimmesdale should have been honest since the beginning so they did did not have to suffer guilt. The moral also means to be true to yourself even when others make fun of you or try to isolate you.