St Pius I governed the Church in the middle of the 2nd century during the reigns of the Emperors Antoninus Pius and Marcus Aurelius. He was the ninth successor of Saint Peter. He decreed that Easter should only be kept on a Sunday. Although being credited with ordering the publication of the Liber Pontificalis, compilation of that document was not started before the beginning of the 6th century. He is said to have built one of the oldest churches in Rome, Santa Pudenziana.
St Pius I endured many hardships during his reign. The fact that Saint Justin taught Christian doctrine in Rome during the pontificate of St Pius I and that the heretics Valentinus, Cerdon, and Marcion visited Rome at the same time, is an argument for the primacy of the Roman See during the 2nd century. Pope Pius I opposed the Valentinians and Gnostics under Marcion, whom he excommunicated.