What does "do that which is good, and no harm shall come to thee" mean in The Crucible by Arthur Miller?
The quote you mention can be found in act three of The Crucible by Arthur Miller. It is spoken by John Proctor as he tries to impart some courage to his servant girl, Marry Warren.
Mary Warren is at the court rather against her will. Proctor is determined to expose Abigail and the other girls as the liars and pretenders that they are, and Mary Warren is his best chance to prove it, since she was one of them. (Plus, if her testimony does not move the court, he will be forced to admit to having an affair with Abigail, and he would of course rather not do that.)
Mary knows this is not going to go well for her, as she has seen Abigail at her threatening worst and knows the other girls will follow her lead. Nevertheless, she tries to be brave, and Proctor tries to help calm her with the words about doing good.
This line comes from a story both Proctor and Mary are familiar with and, when she begins to cry, Proctor leans down and says this:
Now remember what the angel Raphael saint to the boy Tobias. Remember it. “Do that which is good, and no harm shall come to thee.”
Mary remembers the story and is calmed--for now. Later, when she again begins to weep out of fear, Proctor reminds her:
You cannot weep, Mary. Remember the angel, what he said to the boy. Hold to it, now; there is your rock.
Again, Mary is calmed.
This reference comes from the apocryphal Book of Tobit and refers to a story including the archangel Raphael. The words are not difficult to understand outside of that context, however, as it is a simple reminder that doing the right thing (in this case, telling the truth) will keep one from harm.
Unfortunately, Mary is unable to withstand the pressure from Abigail and the girls, and she recants her testimony.