In an effort to show how Abigail is manipulative and a liar, John Proctor finally confesses to having an affair with Abigail in Act 3, Scene 3. When Abby pretends to see spirits and then cries to heaven, Proctor has had enough. He grabs her and calls her a whore. Danforth demands that Proctor prove his accusations, then asks Proctor bluntly, "You--you are a lecher?" Proctor replies, "I have known her" indicating his guilt.
At this point, Proctor chooses to divulge the entire story. John Proctor admits to his affair with Abby which occurred months ago. He also accuses Abby of wishing to be with him after Elizabeth is dead (presumably from being executed as a witch):
She thinks to dance with me on my wife's grave! And well she might for I thought of her softly. God help me, I lusted, and there is a promise in such sweat.
Proctor admits his sin and acknowledges that he understands why Abby might believe that she and Proctor could be together, that Abby might have understood an unsaid "promise" that they would be together after having made love. However, Proctor concludes by shifting the blame from himself to Abby, claiming that, since he rejected Abby, she is exacting her revenge.