John Proctor is talking to his wife, Elizabeth, about the accusations of witchcraft and about his relationship with Abigail Williams when he speaks the words “Because it speaks deceit, and I am honest.” Elizabeth knows that Abigail desires to take John from her, and she believes that Abigail will likely accuse her soon.
Elizabeth also knows that something was definitely going on between John and Abigail. She has seen him blushing when he sees Abigail in church, and she believes that he is ashamed because both she and Abigail are there together. John says that if he were a stone, he “would have cracked for shame” over the past seven months. Elizabeth wants him to renounce Abigail completely and “tell her she's a whore.” If Abigail thinks that John has made her any promises, he must break them at once. He says he will go, but the only promise he gave Abigail is the “promise that a stallion gives a mare.” In other words, they have had sexual relations, and now Abigail thinks John has made a commitment because of that.
Elizabeth again tells John to break that promise and asks why he is angry about that. Here is where John responds, “Because it speaks deceit, and I am honest.” John has never actually made any promises to Abigail. There is nothing to break. The encounter they had seems to have been a one-time thing, and John calls it “the single error of my life” and is frustrated because he cannot put it behind him. He does not want to talk to Abigail about it, because he feels he owes her nothing. He will be admitting to some sort of promise he never made by attempting to break it, and this is dishonest.
Whether or not John Proctor is an honest man is up for debate, but we can see his point about wanting to leave the past in the past. Unfortunately, the past will not let John Proctor go, and the consequences of his affair with Abigail have now come back to haunt him.