This article investigates the reasons for the choice of vernacular language instead of Latin in the communications of bishops with clergy and laity at the end of the sixteenth century and into the first decades of the seventeenth. The spread of Lutheran doctrine, which encouraged the use of the vernacular in the Scriptures and in the mass, was confronted by a reaction: the Catholic Church denied all access to the mysteries of faith to anyone ignorant of Latin through the three Indices of prohibited books issued in the second half of the sixteenth century (1559, 1564, 1596). However, concurrently with the issuing of these prohibitions, the bishops of Italy used the Italian language to translate some papal bulls and decrees of the Council of Trent. On which issues and under what circumstances did they feel it was necessary to be understood by the non-Latinate and therefore find it necessary to supply Italian translations of official documents, such as papal constitutions and Tridentine decrees? Was the local translation of such documents faithful to the original? And if not, why not?