Big fan of

"The point of modern propaganda isn't only to misinform or push an agenda. It is to exhaust your critical thinking, to annihilate truth"

• Garry Kasparov (2015)

That is to say, don't let them shut you up.

There is a longer, more historical version of this but it escapes me at the moment.

I can only find the Hannah Arendt quote (1974)

"This constant lying is not aimed at making the people believe a lie, but at ensuring that no one believes anything anymore. A people that can no longer distinguish between truth and lies cannot distinguish between right and wrong. And [to] such a people... you can do whatever you want."

But i swear there was one that finished "When the truth is silent, liars can move onto action"