Pope Siricius

Pope Siricius (c. 334 – 26 November 399) was the bishop of Rome from December 384 to his death on 26 November 399. In response to inquiries from Bishop Himerius of Tarragona, Siricius issued the Directa decretal, containing decrees of baptism, church discipline and other matters. His are the oldest completely preserved papal decretals. He is sometimes said to have been the first bishop of Rome to call himself pope.

Source: Wikipedia — https://en.wikipedia.org/wiki/Pope_Siricius