I realise that using the term nationalism to describe basically anything before the 17-1800s is rather iffy. I mean how much of a Roman "identitity" or seperatist/restorationist sentiment was there in Greece after the Byzantines fell?
There has been a prior discussion on this which can be found here, answered by /u/The_Turk2
The answer they provide is basically, the term Roman still has not fallen out of use amongst the Greeks - "Rhomaios" or "Roman" is still used casually and there are isolated pockets in Greece (and Turkey) which still call themselves Romans.
But for the most part a Hellenic identity was invented in the 19th century.