Pius I — by ancient tradition the brother of Hermas, author of the Shepherd of Hermas — was elected pope around 140. His pontificate of about fifteen years saw the Roman church receive Justin Martyr at his school in the city, condemn the Gnostic teacher Marcion, and engage in the dating-of-Easter discussions that would continue for the next century.
Tradition holds he established the celebration of Easter on the Sunday following the 14th of Nisan — the Roman position that would prevail at the Council of Nicaea two centuries later. Several Roman titular churches trace their foundation to his pontificate. He was buried near St. Peter on the Vatican Hill.