Trump, in his own mind, is not lying and is telling the truth.

And his supporters agree with him.

Humans are creatures that are capable of rationalizing their beliefs and actions but we are not rational beings.

I'm not condoning Trump or his supporters but am pointing out that people of average intelligence or higher are quite capable of filling their head with utter bullshit that supports their world view and believing, without question, every smelly ounce of it.