John 8:32, essentially says, "Then you will know the truth, and the truth shall set you free."  It means that once a repentant sinner confesses his sins and acknowledges the truth of God and a sin-free life, he will be happier since he will no longer be trapped by his sins.  The judges in "The Crucible" used that philosophy in extracting confessions from people.  The judges convinced people that if they "told the truth", they would be absolved of their sins and would then be able to get into heaven.  The people of Salem had a profound fear of hell, so even the innocent, thinking that confessing to things they had not done they would be assured of going to heaven, were willing to confess to whatever the judges declared they should confess to.  The judges knew and understood this idea and played on it.