John Proctor has known the truth since the very beginning, so why he doesn't choose to speak out and reveal everything to stop the rumor of witchcraft in The Crucible?

John hesitates because he doesn't want the sordid truth of his affair with Abigail to get out. He prides himself on being a loving family man, but that reputation will be ruined if everyone finds out what he's been up to behind his wife's back. One should also bear in mind that John still harbors feelings for Abigail; he knows that ending their relationship is the right thing to do, but his heart holds him back from making a final break with her. Breaking up is hard to do at the best of times, but it's especially hard when there are still strong feelings involved on both sides.
Eventually, John does indeed reveal that Abigail's accusations of witchcraft are nothing but a pack of lies. But by then his sordid secret's already out in the open. And by then, it's already too late anyway because the witch-hunting hysteria has gotten completely out of hand. In the strange topsy-turvy world of witch-craze Salem, it's pathological liars like Abigail who are believed and fine, upstanding citizens like John who are damned as witches.