"[2] 16th-century prelate Antun Vrančić also used the term to embrace all South Slavs, and noted that the people of Belgrade (today in Serbia) spoke Illyrian – ″The local inhabitants who speak the Illyrian language call it Slavni Biograd, which means ‘renowned’ or ‘glorious,’ because of the bravery of its soldiers and officers who after the fall of Smederevo and the Serbian state were able to hold out so long in its defense" – while also applying the term to the language of "Thracians" and "Bulgarians".
[2] Writing in 1592, bishop Peter Cedolini applied the term even more widely: he believed all the Slavs had a single common language, which he called Illyrian.
[2] Jesuit Bartol Kašić, as part of his missionary work, sought to find a common South Slavic language that would be understandable to all.
[2] In 1604 Institutionum linguae illyricae libri duo (the structure of the Illyrian language in two books; 200 pages) was published in Rome.
Bartol Kašić adopted the South Slavic dialect of grammar in Shtokavian, signling out the subdialect of Dubrovnik that was his vernacular.