He was ordained in 1829. During his studies, Cullen acquired knowledge of classical and Oriental languages. He was later appointed to the chairs of Hebrew and Sacred Scripture in the schools of the Propaganda, and receiving at the same time the charge of the famed printing establishment of the Congregation of "Propaganda Fidei". This later charge he resigned in 1832, after being appointed Rector of the Pontifical Irish College in Rome, but during the short term of his administration, he published a standard edition of the Greek and Latin Lexicon of Benjamin Hedericus, which still holds its place in the Italian colleges; he also edited the Acts of the congregation in seven quarto volumes, as well as other important works.