Understanding and updating beliefs on deeply engrained topics can take enormous efforts, but sometimes it can be so hard that the listener cannot even in principle accept the new reality. The listener is simply not ready, he lacks a vast background of reasoning leading to the new understanding.
What fact that you know is true but most people aren't ready to accept it?
By "you know is true" I really mean "you are very confident to be true".
Feel free to use a dummy account.
Candidates that come to mind:
At least, one can say that one should require very, very, very, very strong evidence of the above beliefs before deciding to promote totalitarianism and such.