I adore The Crucible, and I'm actually excited to find a question about it here (I'm sad, I know). One of the only plays that has made me cry!
Abigail Williams and John Proctor have engaged in an affair prior to this. Proctor evidently still has feelings for Abigail, despite everything, and thus has some hesitation towards revealing her as a liar and a fraud. Him revealing such things would also lead to the exposing of their affair, thus exposing his own wickedness. Equally, although he engaged in adultery, Proctor still sees himself as a Christian man. To implicate the seemingly young and innocent Abigail in such business would taint this, both for himself and in the eyes of the court and community. His place in the community is all he's got left, really, and encompasses most of his identity.
