In Act III, Mary Warren has been brought to the court by John Proctor. He wants her to admit in front of the court that the testimony of Abigail and the other girls has been fake all along. This is essentially what she tells Danforth.
Danforth is suspicious of the two of them largely because of what they are there to say. He, not surprisingly, wonders how Mary could be telling the truth now since she had sworn other things before. He is also suspicious because her actions and those of the other girls seemed so realistic to him. Finally, he seems to think that the two of them are just there so as to make him and the court look bad.
Mary Warren is in court because John Proctor has brought her there to submit her deposition and testify that "She never saw no spirits [...]" (Act 3). He is attempting to save his own wife, as well as the wives of Giles Corey and Francis Nurse, all of whom have been accused of witchcraft by Abigail Williams. He also wants the court to understand that Abigail has ulterior motives in her accusations. Danforth is immediately suspicious that Proctor's assertion that he only wants to save his wife is untrue, and so he tells Proctor that Elizabeth is pregnant, and thus will be saved for at least eight more months. When Proctor says that he also wants to free his friends' wives, Danforth with "an almost imperceptible hardness in his voice" says, "Then your purpose is somewhat larger." Since he and Parris believe that innocent people are happy for the courts in Salem, Proctor's dissatisfaction with court automatically singles him out as a potential problem.
Concerning Mary Warren's former statements that people sent their spirits out on her, she now says, "It were pretense, sir." She admits that she lied before, when she accused them, even though she "knew that people would hang by [her] evidence," according to Danforth.
Judge Hathorne instructs her to pretend to faint, as she did before, but she cannot: "I -- have no sense of it now, I --," she cries. Finally, when the girls begin to insist that Mary is sending her familiar spirit to attack them, she calls Proctor "the Devil's man" and returns to Abby's fold.
To Proctor, then, Danforth says, "What are you? You are combined with anti-Christ, are you not? I have seen your power; you will not deny it!" By the end of the act, Danforth is sure that Proctor is in league with the devil to tear down the courts in Salem.