St. Julius became pope in 337, and led the Church until 352. During his papacy, the Eastern Arians were spreading their heresy widely, even though the Council of Nicea had condemned Arianism.

St. Athanasius had come up against the Arians, who had condemned him and contributed to him being deposed. Athanasius wrote to Julius to plead his case and to ask for help, and Julius ruled in favor of Athanasius. In 341, he convened a synod, although the Arians refused to attend, and over 50 bishops agreed that Athanasius had been condemned unfairly.

Pope Julius also helped spread Christianity during his pontificate, building two new basilicas and three cemetery churches. He died on April 12, 352, and was buried in the Cemetery of Calepodius.