Only who can tell? Are you missing a word, or is "only" a person?
It is saying that while the church and court state all liars are damned, it is also encouraging some to lie to the court so the state wont have to kill them for no crimes. So in the view of the writer, God holds liars accountable, however, the real liar is those who take others God given life in the name of truth and justice, and God knows it.
God knows who the sinners are, nothwithstanding the Church and Courts.
One has to remember the times in which Miller wrote this play: many writers were suffering from the state (McCarthy hearings) for what the state held as "evidence" of "sins", when in fact these folks were innocent of any crimes against God.
Reed: Lucy missed the word God in the sentence. << but only GOD can tell who the real sinners are..>>
Hypocrisy is one of the major themes in the play.
Assuming you have seen a film version of Arthur Miller's THE CRUCIBLE, think about what happens in the movie. A woman is accused of a crime (witchcraft). She denies that she is a witch, but the court and the church decide she is lying and is, indeed, a witch. Who is right? The woman, or the court and the church? Only God can know. Maybe she is lying, or maybe the court and the church (human institutions) are wrong. Only God knows for sure. Should we be so quick to pass judgement and punish someone for being a witch, or leave such matters to God.
If she had been accused of theft and there was ample evidence that she is guilty (she had the stolen property in her possession, someone saw her do it, etc.), that's one thing. Being a witch is something no one can see. But God can, if we can't.