Gregory XIII
Corpus Juris Canonici and Gregory IX, Decretales. Coloniae Munatianae : sumptibus fratrum Detournes, 1779. Folio (2 vols). Full calf, spines gilt in compartments. The Decretals of Gregory IX are the critical source of medieval Canon Law. In 1230, Pope Gregory IX ordered his chaplain and confessor, St. Raymond of Penyafort, to form a new collection - probably to reinforce the power of the Papacy, then at its zenith. Unlike most bodies of law, or collections of laws, all decisions were deemed canon law (whether or not authentic); all were simultaneously promulgated, and thus are equally obligatory (even when contradictions arise) and have equal primacy; and it is exclusive, abrogating all previous collections later than Gratian's Decretum. It was the single most important revision of canon law, still forming the basis for the modern corpus.

This edition was published in the early years of Pius VI's Papacy. The power of canon law will have been of particular interest to Pius given his early reformist stance, opposition to corruption, and the challenges to him from protests seeking to limit temporal Papal authority. Pius would end his life in France. After his opposition to the French Revolution, French troops occupied Rome, declared a Roman Republic and demanded Pius renounce his temporal authority. On his refusal he was taken prisoner and eventually transported to Valence, where he died.