On this we agree. This truth needs to be on a scale of one, some, or all truths that are shared. Let's disqualify all, and simplfy to finding an example of one.
The other thing about the truth is that it probably shouldn't be falsifiable, as it's a bit of an all-in bet on whatever that is. It shouldn't be a complicated mystery because then you are back to the relative problem of who has discovered more and how do you know. The perfect example truth is the one that is not knowable either way, but something that can be believed.
We should probably recognize that destabilizing that example shared truth is an attack on social cohesion as well. Can't think of an example at the moment, but I certainly agree it would be the One Important Thing you would need to believe, or you're basically left with a kind of hellscape.
The fact is that the earth is (nearly) round, contrails are not chemical sprays, Capricorn One was a C-grade movie while Apollo-11 went to the moon, and that anti-vax movement was started by a fraud attempting to discredit existing formulas to promote a formula in which the paper's author had a vested interest (and which paper was retracted, along with his MD and license) and that vaccines do work.
That is the point. A society cannot remain cohesive if there are no shared truths.
You may not want to take it seriously, but I'd suggest you consider the following quotes, and why and how disinformation can be weaponized.
“Anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge.'” ― Isaac Asimov
"You are entitled to your opinion. But you are not entitled to your own facts.". ― Daniel Patrick Moynihan.