Answer:
This question requires a personal answer, since it must be based on some work that you have read.
Anyway, I will give you an answer so that you can take it as an example and thus analyze a work that you have read and believe appropriate for this quotation.
Step-by-step explanation:
Young Goodman Brown by Nathaniel Hawthorne
If we analyze this quote we can interpret that it refers to the character of each person inside, or in their privacy.
One can show their values or how their character is formed but what really counts is who you are inside.
This applies to the Young Goodman Brown story.
Here we can see how a whole city of "good" people is tempted by the devil, including Goodman Brown.
They are all practitioners of the puritan religion, and even though they seem to be exemplary people, such as the deacon of the church, we know well that they are not.
So, the character of these people is not formed by what they appear to be, but by what they are in the dark.