John Proctor suddenly and dramatically reveals his adultery to the court in Act III of The Crucible with the words:

I have known her, sir. I have known her.

This rather euphemistic way of describing sexual intercourse is Biblical in origin and therefore doubly appropriate for the Puritans. Arthur Miller says that Proctor is “trembling, his life collapsing about him” as he utters the confession.

Proctor has to confess to substantiate the charge he has just made against Abigail. When she cries aloud to God to “take away this shadow!” Proctor responds:

How do you call Heaven! Whore! Whore!

Having called Abigail a whore, Proctor has to justify himself. Of course, she is not technically a whore (as Miller’s Epilogue suggests she may go on to be), but she is a woman guilty of fornication, and Proctor can best charge her with this by indicting himself.

Once he has made his initial confession, Proctor goes into some detail. He says that they had sex “In the proper place—where my beasts are bedded.” He then asks Danforth for forgiveness, though this is surely not in the Deputy Governor’s power to give. He concludes his confession with the words:

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.

Though he shows true contrition, particularly for the pain he has caused Elizabeth, Proctor’s continual abuse of Abigail as a whore shows that his desire to blacken her name and take away her power is the primary motivation behind his words.

In act three, John Proctor sacrifices his positive reputation and good name by publicly admitting that he had an affair with Abigail Williams in hopes of undermining her authority and proving that the girls are frauds. After Abigail begins to act like Mary Warren's spirit is attacking her, John Proctor calls her a whore and admits his infidelity by telling Deputy Governor Danforth,

"I have known her, sir. I have known her" (Miller, 110).

Danforth responds by directly asking Proctor if he is a lecher and Francis Nurse is astonished by John's confession. Proctor then proceeds to elaborate on his affair with Abigail Williams by giving specific details about their sexual encounter. He tells the court officials that the affair took place about eight months ago and that he slept with Abigail in his barn, where his "beasts are bedded." Proctor goes on to explain that Elizabeth kicked Abigail out of their home following the affair and Abigail wishes to dance on her grave, which is why she falsely accused Elizabeth of witchcraft.

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. 

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.