I think it’s useful to think about the Roman Empire.

First you had Latin. As the Roman Empire expanded through Europe- Spain, France, Portugal, the rest of Italy, more and more people adopted the Latin language as their own. Of course the Latin’s weren’t all the same.

Imagine for example:
A Jamaican that speaks Oxford English
A Jamaican born and raised in New York
A Jamaican that moved to New York as an Adult
A Jamaican in Jamaica doing a news report
A Jamaican talking in a classroom
A Jamaican talking to his friends or family
A poor Jamaican farmer talking to his friends

Now imagine the same thing from a Scottish person.

You’ll see that in each scenario we’re dealing with an English that diverges more and more from what we might call standard until we get to something that we might even call Jamaican Patois (or Scots).

Now to some degree even at their extremes, Patois and Scots these are both English. Both the poor scots farmer and poor Jamaican farmer understand English. But do they understand each other?

The Latin of the Roman Empire existed on such a continuum. Eventually these variants of Latin (Latin and Vulgar Latins) evolved and diverged overtime to what we know today as French, Spanish, Italian, Portuguese. They are in some ways one language. But in other ways separate languages.

Well for Arabic, the classical Arabic is like the “Chaucers Arabic”. It’s regarded as being pure. But what you hear on the news in Arabic is more like Shakespeares Arabic. Close. Any English speaker can understand and although it takes some effort it takes a lot more effort to be able to read Chaucer. What you hear in the streets however, is essentially the Scots, and Jamaican Creoles of Arabic. It’s all the same language, kind of, but at the same time it’s pretty different. Usually if an Arabic Speaker from say Morocco and Yemen want to have a conversation- they aren’t going to be able to do so effectively in their own “Creole/Scots”. Some people would have you believe that they will adopt the Standard “Shaespeare” Arabic found in the news. But this isn’t even true. What happens more often than not is they mix their own dialects with Egyptian Arabic which isn’t terribly too different from how most people learn American English these days even though they regard British English as being more pure in a sense. Egypt is the media powerhouse of the Arab world.

Written Standard Arabic then is sort of like me asking you to write using Shakespeare’s English in that it’s going to differ greatly from your normal way of talking (Scots/Creole) whereas classical Arabic is like Chaucer’s English in that…with education you can read it but you’ll probably have a hard time imitating it well even though it’s sort of similar to Shakespeare.

So if I would describe Arabic today, I’d say the situation is essentially like how in the 1500 you had all these French Spanish Italians and whatever still writing in Latin while the languages they all spoke were becoming more and more divergent over time.

An Arab speaker could probably talk to Mohammad but it’s likely Mohammad would have a hard time understanding the Modern Arab speaker. It's sort of like asking Shakespeare to read your text messages.