St. Siricius
Siricius · 384 – 399
Issued the first surviving papal decretal and began shaping the papacy into a legislative authority.
Siricius became pope in 384 and did something new: he answered a bishop's questions with a formal letter that read like a legal ruling, complete with binding instructions. This was the first papal decretal, and it changed the nature of the papacy. Previous popes had taught, preached, and adjudicated. Siricius legislated. His decretals covered clerical celibacy, baptismal practice, and the readmission of heretics. He required celibacy of priests and deacons, a policy that has remained in the Latin Church ever since. He also clashed with Jerome, whose combative personality made enemies easily. Siricius governed for fifteen years, one of the longer pontificates of the era.
The Pope as Lawmaker
Before Siricius, popes mostly led by example and correspondence. Siricius started issuing formal rulings that bishops were expected to follow. It was a significant step toward the centralized authority the papacy would later develop. The decretal format he pioneered became the standard way popes communicated binding decisions.
Celibacy as Law
Clerical celibacy had been a widespread practice, but Siricius made it mandatory. Whether you think that was wise or not, it has shaped the Western priesthood for over sixteen hundred years.