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. In 1574 Ivan Petar Kašić married for Ivanica Bogdančić and they had a son Bartol next year. 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. He attended the municipal school in the town of Pag. After 1590 he studied at the Illyric College in Loreto near Ancona, in the Papal States (in modern Italy), managed by the Jesuits. As a gifted and industrious pupil, he was sent to further studies in Rome in 1593, where he joined the Society of Jesus in 1595. Kašić continued propaganda activities of Aleksandar Komulović after his death, being even greater Pan-Slav then Komulović was. Kašić censored and edited Komulović's 1606 work ("Zrcalo od Ispovijesti").