In Act III of "The Crucible," John Proctor admits to having an affair with Abigail. Abigail denies it. Danforth is about to bring Elizabeth Proctor in to testify. Before he does this, he asks John if Elizabeth is an honest woman. John says, "In her life, sir, she have never lied." Danforth asks John if they kicked Abigail out of their house for being a harlot (having the affair with John). John says that this is true.
Elizabeth Proctor is brought into the court. Elizabeth does not know John has confessed to adultery. When questioned, she says he is not a lecher (did not commit adultery). By this point, John has lost credibility, having claimed that Elizabeth never lies. Either she is lying or he is. Danforth believes John is lying.
In addition, Abigail's assertive attitude and superb acting/lying has been consistent. Ironically, this makes her (seemingly to Danforth) more reliable because of that consistency. Even though Hale believes John, Danforth concludes that Elizabeth was being truthful and John was lying. To further establish herself as the victim, Abigail pretends to be attacked by Mary's spirit. At this point, Mary also turns on John to save herself, saying "I'll not hang with you. I love God, I love God."
Once again, in spite of Hale's defence of John, Danforth has made up his mind that Abigail is the victim and John is the villain.