The issue of truth is very much at the heart of The Crucible. The rampant witch craze that has descended upon the town of Salem like a plague of locusts is based on the very negation of truth as most people would understand it. Absolutely no one is guilty of the crime of witchcraft, and yet, thanks to the bare-faced lies of Abigail Williams and her petrified cohorts, the finger of suspicion is pointed at countless innocent people, many of whom are sent to their deaths as a consequence.
It's often said that the truth will out, but that's not what happens here. In fact, the bigger the lies told by Abbie and the other girls, the more likely people are to believe them. That's not to say that everyone in town is ignorant of the truth; John Proctor certainly knows what's what, as indeed does Reverend Hale, eventually. But in such a feverish, hysterical atmosphere, none of this matters. Lies have been established as the norm, and the truth is in constant retreat.
Under the circumstances, it takes enormous courage to stand up to such a sustained barrage of untruth. But for one reason or another, no one, other than John Proctor and Reverend Hale, are prepared to do this. Most people in town have been cowed into submission; they're too frightened to stick their heads above the parapet and challenge the mass hysteria that has fallen upon the town.
It takes real courage to stand up against such madness: the kind of courage that John Proctor displays and which will cost him his life. But in the end, John's sacrifice is futile, because the witch craze looks set to continue long after he's been executed and his body lies moldering in the grave.