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.