We can decipher a lot of great thought, even from before 1850.
Reach eg Archimedes' "The Sand Reckoner" (http://euclid.trentu.ca/math/sb/3810H/Fall-2009/The-Sand-Rec...).
Some meta-information about it at Wikipedia: https://en.wikipedia.org/wiki/The_Sand_Reckoner
Freud is perfect example. He caused huge revolution in the philosophy of mind and changed our culture in many visible ways, but his theories did not survive.
Freud's theories are based on outdated concepts of how the brain works. The evidence was usually incomplete or questionable. Freud based his theories on anecdotes from his practice and his patients were mostly upper class women who suffered from sexual repression. Freud overgeneralized from this very odd set of patients very strange theories.
Still his basic themes are considered correct even today and they were revolutionary at the same way as Einstein's. They seem so basic and obvious to us that we don't think there was controversy or revolution involved.
Five still valid _general_ ideas that are left from his psychodynamics theories after most was debunked:
1. There are unconscious processes. Parts of cognition, motivation and emotions can be hidden from consciousness.
2. There are competing processes and conflicts in the brain.
3. Personality is significantly shaped by childhood
4. Our mental representations of other people shape our social relations.
5. There is a sequence of different developmental stages that create internal psychological conflicts.
http://psych.utoronto.ca/users/peterson/psy430s2001/Westen%2...
People have always been people, and most of their concerns and theories were more or less similar. Even the most alien ones can still be relatable (in fact that's already an ancient, roman, idea: "Homo sum, humani nihil a me alienum puto", by Terence).
Until very recently (up until the seventies or so) when humanities still reigned supreme in universities and schools, being able to read, enjoy and understand anything from Homer and Plato to Shakespeare and Kant was nothing special for higher education people.
That said, often being able to parse those things is question of being accustomed not to an era, but to a way of thinking. (E.g. Anglo-saxon vs continental philosophy).
My favourite example is one of Robert Musil's essays. There are multiple versions of it, showing that he desperately tried to convey a thought and make it as precise as possible, but the meaning escapes me.
The reason for fascination is that even in the past there must have been clever people, Einsteins born 1259, that had valuable thoughts but were able to express them only within the contemporary context, using contemporary means.
It may even be that those thought could possible lead to whole new areas of knowledge but were never followed simply because the world have moved in a different direction in the years that followed.
A sad thing is that these alternative thoughts can be spotted only if they occurred not that long ago (for me, personally, the limit is maybe 1850). When you go further into the past, the things get simply weird and undecipherable (angels dancing on the tip of the needle, anyone?) Thus, the possibility to explore alternative branchings of the river of thought disappear forever as we move forward.