He is notable for asserting the authority of the pope over the Arian Eastern bishops, as well as setting December 25 as the official birthdate of Jesus.

Julius was a native of Rome and was chosen as successor of Pope Mark after the Roman seat had been vacant for four months.

After the followers of Eusebius of Nicomedia, who had become the patriarch of Constantinople, renewed their deposition of Athanasius of Alexandria at a synod held in Antioch in 341, they resolved to send delegates to Constans, emperor of the West, and also to Julius, setting forth the grounds on which they had proceeded.

Julius, after expressing an opinion favourable to Athanasius, adroitly invited both parties to lay the case before a synod to be presided over by himself.

The three hundred Western bishops who remained, confirmed the previous decisions of the Roman synod and issued a number of decrees regarding church discipline.

[4][5][6] At the time this was one of the commonly believed dates for Jesus' birth and was used by Hippolytus of Rome in his Commentary on Daniel around 200 AD.

[7] It is claimed – falsely – that Pope Julius declared December 25 as Christmas after patriarch Cyril of Jerusalem asked for clarification on what date historical records stored in Rome indicate as Jesus' birth.