I assume you will be writing an essay on The Scarlet Letter by Nathaniel Hawthorne which focuses on the devastating effects of hidden sin. I also assume your primary example will be the Reverend Arthur Dimmesdale and his miserable existence because he harbors hidden sin which eventually destroys him. Remember that there are others in this story who harbor hidden sin, including Roger Chillingworth.
This quote is the warning, the moral of the story, and it suggests that confession, in some form, is what will allow you to be true to yourself and save your soul.
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!
It would help to know what, exactly, you wanted to write about, but perhaps you are really looking for ideas as much as just a thesis sentence. So, here are some possible thesis sentences which can be tied to this quote.
- Hester is able to find peace with herself because her sins were known by all; Dimmesdale can only find peace after he reveals his hidden sin. (This essay compares the two of them and how they handle their sin.)
- Arthur's internal suffering was made worse by the fact that his parishioners saw him as being perfect, and he did nothing to change their view of him. (Of course this references his "confessions" which he knew the people would never accept and, even worse, would make him look even more pious in their eyes.)
- If Arthur had confessed his sins while Hester was on the scaffold, his life would have been better. (This essay would outline his miseries, specifically beginning on the day he begs Hester to help him by speaking his name as her "partner in crime," and ending with his confession and freedom, also on the scaffold, at the end of the story.)
- The same sin that eats at Arthur's conscience internally also appears physically in and on his body. (There is plenty of evidence that as the weight of Dimmesdale's guilt grows stronger, his body grows weaker--and there is also the "his own red stigma" which "has seared his inmost heart.")
- Because he hides his evil intentions from others, no one connects Roger Chillingworth's physical deterioration with his evil desire for revenge; however, the connection is clear.
- Puritanism encourages people to hide their sins in order to avoid harsh, intractable punishments; two examples of that include Arthur Dimmesdale and Roger Chillingworth. (This is a chance ot discuss the poisoned, hidden connection between these two men.)
The novel is replete with examples of the concept of hidden sin and its effects, so choose some aspect of it which appeals to you and with which you can find effective examples. Remember, too, that one way to define or describe something is by saying what it is not. Anything that contrasts or explains open, known sin and hidden, festering sin will suit this quote.