You cannot disprove the allegation that Julius Caesar was in, in fact, an alien cyborg sent here to change the course of history. You can't produce a body, nor is there any historic document expressly saying that he was not an alien cyborg.
If you say that he was not an alien cyborg in your opinion that's okay, but if you announce that he wasn't an alien cyborg as a fact, you're thick, because obviously, you can't prove that.
Sounds a bit silly doesn't it?
Trying to prove a negative is insanely hard, especially when there's virtually nothing to actually look for/investigate with this kind of thing. Believing in something just because it cannot be dis-proven is an iffy way to go about things, simply because it gives the benefit of the doubt to literally anything and everything without evidence to the contrary.
Believing in something is just fine, I think. But belief without evidence is just that, a belief. That's not to belittle belief and faith, they make up a bit part of this world, but to call people thick because they cannot disprove your beliefs or disagree with them seems a bit extreme.