Before the printing press was a thing and books where copied by hands by scholars, anyone dedicated enough could have been a theoretical know it all.
These days with the amount of information that the human race generates, you can barely get 5-10% of all the knowledge on a single field, and my numbers are probably an exaggeration, but for example I'm a software engineering student and I know it's impossible for me to know 10% of everything out there in my field, even if I dedicated all my life to it, new knowledge gets generated faster than the speed I can proccess or anyone else can processes current day knowledge.
That's for sure. There's no way to keep up with even a fraction of it now.