What Does John Proctor Mean When He Says "God Is Dead"?

SparkNotes explains that in Act III of "The Crucible," John Proctor is put to the test when he decides he must confess to his affair with ringleader Abigail Williams. After being unjustly accused of witchcraft, he screams, "God is dead!" to show that if he is accused of witchcraft, there must not really be a God.