Saint Julius I was elected Pope on February 6th, 337, after the chair of the Bishop of Rome was vacant for a full four months since the death of Pope Arcus in October of the previous year. When Constantine the Great died in May, his son Constantine II assumed the Emperor's seat and allowed Athanasius to return to the See of Alexandria. Athanasius had previously been banished by Arians who had told the Emperor that Athanasius would cut off grain shipments to Rome from Alexandria if he was allowed to remain as Bishop. Constantine found him guilty and had spent several years in exile in Augusta Treverorum in Gaul.
Eusebius of Nicomedia, the then Patriarch of Constantinople, and his followers renewed their disposition of Athanasius at the synod in Antioch in 341. They sent delegates to the Emperor of the Western Empire and to Saint Julius with letters explaining how they had reached their decision. Saint Julius immediately sent out letters of invitation for a Synod where evidence could be presented by both the Eastern Bishops and the Arian Bishops regarding Athanasius. The Arian Bishops refused to participate and instead held their own council at Antioch where they elected George of Cappadocia as the Bishop of Alexandria. Athanasius was again forced into exile. Several other Eastern Bishops, having been removed by the Arian members at the council, travelled to Rome. Saint Julius found their profession of faith satisfactory and re-established their episcopal rights.
During his second exile, Athanasius came to Rome. Saint Julius convened a second synod in 342 where he recognized Athanasius as a full Bishop and sent letters to the Eastern Bishops showing primacy of the Bishop of Rome. In these letters he laid the ground for how he found Athanasius innocent and derided the church in Alexandria for now following custom inwriting to the Pope before making their decisions. Saint Julius wrote:
"Can you be ignorant that this is the custom, that we should be written to first, so that from here what is just may be defined" (Epistle of Julius to Antioch, c. xxii)
Saint Julius helped convene the council of Sardica in Illyria. Seventy six of the Eastern bishops who attended withdrew to Phillippopolis and held their own council where as the three hundred Western Bishops who did remain confirmed both the previous Roman Synod decisions and made several new decrees relating to discipline within the Church. When George of Cappadocia died, Saint Athanasius was restored as the Bishop of Alexandria (346). Saint Pope Julius took the occasion to write a letter to the priests, deacons and faithful of Alexandria congratulating them of the return of their Bishop.
Saint Julius went on to have several churches erected, the now Saint Maria basilica in Trastevere and the Basilica Julia (now the Church of the Twelve Apostles). He also constructed three churches over the cemeteries outside the walls of Rome: one on the road to Porto, one on the Via Aurelia and one on the Via Flaminia at the tomb of Saint Valentine. The Roman feast-calendar of Philocalus was created during this time and heavily used by Pope Julius during his pontificate. Saint Pope Julius died on April 12th, 352 in Rome. he was buried in the catacombs of Calepodius on the Aurelian way. Sometime later, his body was moved to Saint Maria basilica.
His feast is now celebrated on April 12th.