In Act III John Proctor has brought Mary Warren with him to the court. He feels with his testimony and Mary Warren's deposition, the courts will have to believe them and release his wife.
Unfortunately, the courts are not willing to listen to their evidence, and, just as Mary Warren predicated Abigail and the girls, begin to turn on her. Proctor tries to comfort her. By telling her "Do that which is good and no harm will come to thee" he is trying to convince her to hold tight to their cause. He believes if they stick with their story everything will turn out okay. However, as the girls turn on her, mock her, and pretend to be afflicted by her, there is nothing else she can do. Eventually she gives in and calls Proctor the "devil's man" and returns to the safety of Abigail.