Recently, I have started to question this myself.
Interestingly, I think proof in itself is subject to the limit of understanding and believe of the subject.
Scientific proof is a great example: many theorems have been proven scientifically or mathematically, but unless you are able to grasp and understand this proof, you as an individual must take the word of another person that this is, in fact, the case. That is taking something on faith, isn't it?
Now, within our modern society, this becomes especially problematic. People seem to readily embrace their own ideas, and disbelieve certain truths that have been established by scientists. They might do so because of religious believes(evolution or universal vaccination), or, as the cynic in me fears, expediency(climate(global warming)), or just out of general conviction (fake moonlanding).
So they take what proof is out there, and toss that aside, actively disbelieving it. Given how they can do that with relative impunity, I have to wonder what proof is really worth nowadays? If certain 'proven' facts are not to a person's liking, (s)he can just toss them aside and replace them with their own worldview.
So have we now arrived in a society where proof counts for nothing, and reality is infinitely malleable to conform to whatever set of 'truths' you wish to frame it in? My mind boggles at the concept... (but then, given that it isn't all that much of a mind to begin with, it's fairly easy to boggle it, really).