What is "the truth"? Whose truth? They for sure have their truth. No matter what you tell them and show them they will still have their version of "the truth". The only way for them to realize their version is based on lies is want to test their knowledge and evidence. But they cant, they are not allowed to, they are fearful.
Unless they are willing to face the consequences of researching, of testing, of questioning and doubting they wont be able to see anything other than what they believe.
Even if all the GB members were Dfd and replaced with new ones they would think that God is replacing them.
The truth starts by realizing what is the kind of "truth" they have.