When you discover someone has been lying to you, you start to treat everything they say as lies.
I used to believe in 9/11.
Then the same people who brought you that story discovered that Iraq had Weapons of Mass Destruction.
Anyone who had observed Iraq for a long enough time knew that was false.
That gets you to start to treat all their other claims as lies until proven true.
(Btw this is the same reason I treat antis' claims as false)