Plato famously maintained that knowledge is “justified true belief,” meaning that to claim the status of knowledge our beliefs (say, that the earth goes around the sun, rather than the other way around) have to be both true (to the extent this can actually be ascertained) and justified (i.e., we ought to be able to explain to others why we hold such beliefs, otherwise we are simply repeating the — possibly true — beliefs of someone else).*
It is the “justified” part that is humbling, since a moment’s reflection will show that a large number of things we think we know we actually cannot justify, which means that we are simply trusting someone else’s authority on the matter. (Which is okay, as long as we realize and acknowledge that to be the case.)