Bartol was born in Pag, in the Republic of Venice (in modern Croatia) of his father Ivan Petar Kašić who participated in the 1571 Battle of Lepanto and mother Ivanica.

[1] His father died when he was a small child, so he was raised by his uncle Luka Deodati Bogdančić, a priest from Pag, who taught him to read and write.

In 1612/13, disguised as a merchant, he went on a mission to the Ottoman provinces of Bosnia, central Serbia and eastern Slavonia (Valpovo, Osijek, Vukovar), which he reported to the pope.

Since the Jesuits took care of the Christians in the Ottoman Empire and tried to teach in the local language, they needed an adequate textbook for working among the Croats.

In 1582, Marin Temperica wrote a report to general Claudio Acquaviva in which he emphasized the importance of the Slavic language understandable all over the Balkans.

[8] Based on this request, Kašić provided such a textbook; he published Institutionum linguae illyricae libri duo ("The Structure of the Illyrian Language in Two Books") in Rome in 1604.

In late 1627, he completed the spiritual tragedy St. Venefrida, subtitled triomfo od čistoće (a triumph of purity), which remained in manuscript until 1938.

Kašić's Croatian translation, even incomplete (some parts of the Old Testament are missing), has around 20,000 different words – more than the English version and even more than the original Bible!

Ritual rimski ("Roman Rite", 1640),[clarification needed] covering more than 400 pages, was the most famous Kašić's work, which was used by all Croatian dioceses and archdioceses except for the one in Zagreb, which also accepted it in the 19th century.

He used the term "Bosnian" even though he was born in a Chakavian region: instead he decided to adopt a "common language" (lingua communis), a version of Shtokavian Ikavian, spoken by the majority the speakers of Serbo-Croatian.