Saint Justin taught Christian doctrine in Rome during the pontificate of St Pius I but the account of his martyrdom indicates there was no Roman bishop present there, an unsurprising occurrence, considering the brevity of the account. The heretics Valentinus, Cerdon, and Marcion visited Rome during that period. Catholic apologists see this as an argument for the primacy of the Roman See during the 2nd century. Pope Pius I is believed to have opposed the Valentinians and Gnostics under Marcion, whom he excommunicated.