In Act Three, what is the significance of the quote, "Do that which is good, and no harm shall come to thee"?
In Act Three, Proctor presents a paper to the courts that contains the signatures of people who are ready to declare their "good opinion" of Rebecca, Proctor's wife, and Martha Corey. All of these ninety-one individuals are farmers and members of the church who have had no "dealings with the Devil" and are, thus, in good standing in the community. Mary Warren has also been brought forward to testify that she and the other girls "afflicted" by the witchcraft were only pretending. Mary is emotionally disturbed by the court's proceedings, and Proctor attempts to calm her:
Now remember what the angel Raphael said to the boy Tobia. Remember it. 'Do that which is good, and no harm shall come to thee.'
Proctor optimistically believes that the truth will come to light by doing good and that Mary (and all others who fight on the behalf of truth) will be protected. Unfortunately, in a community overrun by hysteria, this is not true.
Though Mary tries to stand against the other girls by admitting their actual wrongdoings (their lies about the witchcraft) and Proctor himself confesses his affair with Abigail, their truth-telling and "goodness" is not enough to save them from punishment. Mary is terrorized by the girls into submitting to the hysteria once more, and Proctor is accused of being a man of the devil, resulting in his arrest.
The Crucible is a tragedy because the rules that are usually followed, the rules that we count on to be followed are not. In fact much of what normal people accept as true is inverted. Especially the fact that those who are good are being put on trial and killed for crimes they have not committed. And so the staement "Do that which is good, and no harm shall come to thee," is false.