I wouldn't dignify that with the word "truth", and I don't think you should, either ("you" meaning sykh, not "you" meaning the person in your thought experiment).

This person has a very strong belief, so strong that they put the label "truth" on it. But that's a different meaning than the word has had in this discussion thread - so different that different words should be used, to keep us straight. The version you mention here might be called "strongly-held belief" - held so strongly, in fact, that the person holding it can't imagine that it could be questioned, let alone false.

But for the rest of the comments, I think that people are using "truth" to mean "correspondence to reality". I think that that is quite different from "strongly-held belief", because I don't think that strongly-held belief changes reality.

I for instance believe that 1+1=2 under the first order Peano axioms of arithmetic. This belief of mine is so strong that it would be imposssible to convince me otherwise. This is a truth for me (and for every other sufficiently educated person). But there are differing levels of assuredness in our beliefs. Some not as strong as others. Therein lies the rub when one talks about “reality”. Isn’t reality ultimately person and perception dependent? Are there objective, universal truths?

For me a strongly held belief is more or less equivalent to truth for the person who holds such a belief.

There are varying levels of assuredness, but they also line up pretty nicely with the varying levels of experience that people have. If you have more experience verifying something to be true, then that indirectly serves as justification for the beliefs that everyone else has, even if they are not personally so sure.

The truth is a telescope of verifications that trace back through history. It's not a personal thing. It doesn't stop at your skull.

Incidentally, the only time I ever hear people saying truth is relative or doesn't exist is when those people are simply trying to promote their own social standing or take advantage of people.

The personal thing is what a person perceives as truth. Everyone has a set of beliefs that they consider true. The set isn’t the same for everyone. Isn’t it the case then that truth, at least some portion of it, is person dependent?

No, it certainly is not. That only means that one's knowledge is limited, not that the things you have knowledge of are inherently subjective.

The fact that two people disagree about something does not automatically imply that they are both wrong, only that at least one of them is.

I think what you are describing is dogma, not truth.

How is that distinguishable from truth from the perspective of the one who holds said belief?