You are welcome to your opinion, but I'm pretty sure Trump does not "honestly believe that what he says is true". I don't think he even thinks about it...I think he is either blatantly lying or just spewing forth bullshit.
I tend to agree, in many cases, he is just parroting what others say because he 'thinks' it supports his case. Take the recent Sweden riot case, where he saw a Fox propaganda show, mistook it for serious journalism and thought it supported his case where in actual fact it had little if anything to do with his point, maybe even proofs the opposite. A similar thing with crime rates by (illegal) immigrants in the USA, they are in fact lower than the crime rate by US citizens. But facts be damned if they are unraveling the phony fictions he has been peddling.
I've come across an interesting book, as far as the reviews suggest (I have it on order to read and form an opinion of it myself), called
"What Is Populism" by Jan Werner Müller. This Professor of Politics at Princetown University, analyses the current rise of Populism in the USA and Europe and he tries to define it more clearly. His conclusions (plural), seem to point in the direction that populism is anti-democratic and leads to partisanship.
With the lessons learned from history, and although I'm not old enough to have personally suffered from WWII but my parents did, there are just too many parallels with events in those years to not be
very concerned and worried. Any anti-democratic movement in society/politics raises big red flags with me, and that has nothing to do with political preferences because populists come in all flavors, left wing and right wing ... The only thing they have in common is that they are a threat to Democracy while
claiming that they are speaking on behalf of the silent disenfranchised 'majority'. When in power, they further destroy social cohesion and they promote division and partisanship. They thrive on chaos because they can claim that 'the elite caused it' (no need t prove it, facts be damned) and only they can fix it (if they do not have to play by the rules their opponent have to). Absolute power leads to absolute corruption.
You can not dispute that Trump, when fact checked by any reasonable manner repeatedly and consistently says things that are not true. Right? Can you in good conscience dispute that? So, then if we can agree that a lot of what Trump says isn't proven out to be true, we need to figure out why he can't be trusted to tell the truth.
It looks from a distance that it is a mix of pathology and populism (inspired by his alt-right advisors). A dangerous cocktail.
Cheers,
Bart