What does it mean to know something? Does it mean you’re an expert and an authority in that subject? Or simply that you have some level of competence? Or does it just mean a certain degree of acquaintance?
What does it mean to know a language? Does it mean you can pass yourself off as a native speaker? Or is it enough if you can have a conversation with someone without making so many mistakes or mispronouncing the words so badly that you cannot get your point across? Is it enough just to be able to follow conversations? Would it count as knowing a language if you had a lot of vocabulary but you could barely fashion the words into actual sentences? Does it count as knowing a language if you can read it but not speak it, or if you can get by in stores or restaurants but not tell someone your life history?
Let’s imagine that knowing a language means being able to pass yourself off as a native speaker. In that case: what level of a native speaker? We surely can’t expect you to pass for a highly educated native speaker, can we? Because then you’d be held to a higher standard than the average speaker of the language. That would cut out a lot of actual native speakers. Would you have to be able to communicate at the level of a high school graduate? What if you could read the international-news section of a newspaper in some language, but you couldn’t make sense of the local version of Dear Abby?
The point here is that there are many different aspects of knowing a language (speaking, writing, listening, reading, conversing) and many different levels of competence. And that’s true of Latin, as well.
At the highest level—that is, using the most stringent standards—knowing Latin would mean being able to comprehend (whether by reading or listening to) any existing Latin text ever written on any subject, spanning more than two millennia and including every field in which Latin has ever been used, which is all of them, from the most abstract philosophy treatise down to the raciest graffiti unearthed in Pompeii. But it is no more possible to find a person capable of dealing with all this than it would be to find someone who could comprehend any text written in English, whether the content involved physics, philosophy, technology, poetry, any sport at all, medicine, literary prose, self-help, how-to, drama, etc., and whether the text was written in American, British, Indian, or some other English, and whether it was written at the time that Beowulf was produced or just this morning or any time in between.
So, before we block ourselves off from the study of Latin because it’s so obviously impossible to actually learn it, let’s cut ourselves some slack and see what exactly it is a person might want to get out of studying Latin and what level of Latin learning might be enough.