I would think that you want to turn to the last section of the Third Act. It is here where Proctor has to admit to adultery, if nothing else to try to blunt Abigail's meteoric rise to power. Proctor admits to adultery as he recognizes that Mary Warren's testimony is not going to be effective. John recognizes that coming forth in this forum about his adultery is the last refuge he has:
Excellency, forgive me, forgive me. 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! But it is a whore’s vengeance, and you must see it; I set myself entirely in your hands, I know you must see it now. My wife is innocent, except she know a whore when she see one.
Another moment in the scene where he confesses to the sin of adultery happens at the end when Elizabeth is brought in for questioning. She lies in thinking to defend her husband and before she leaves, he cries out, "Elizabeth, I have confessed it." In this, one sees a clear revelation of adultery. In confessing, Proctor is operating on the level of self- awareness and full disclosure being the only way to remedy the hidden demons that are holding the people of Salem hostage to speaking the truth.
In Act Three, John Proctor goes to the court with his friends Giles Corey and Francis Nurse in order to provide evidence to prove their wives' innocence. He has not revealed his affair with Abigail Williams publicly because he knows the terribly detrimental effect this news will have on his reputation. However, he knows that there is a good chance he will have to confess to the affair in order to prove that Abigail has ulterior motives for accusing his wife of witchcraft. He yells—in sheer desperation and frustration because the magistrates so clearly believe Abigail's lies—"It is a whore!" Danforth doubts him and Abigail denies it. Proctor says, by way of explanation, "I have known her, sir. I have known her." Although the meaning of this language might not be immediately obvious to us, Proctor means that he has known Abigail in the biblical sense, and Danforth understands this meaning.