12 April - Julius of Rome or Julius I, born in Rome around 280 and died in the same city in 352, is a bishop of Rome who acceded to the episcopate on 6 February 337. According to Catholic tradition, he is the 35th pope.

After the ephemeral episcopate of Mark, the seat remained vacant for four months for an unknown reason. Julius, a native of Rome and son of a certain Rusticus, was elected on February 6, 337, the year of the death of Emperor Constantine. The creation of the Apostolic Chancellery, the oldest dicastery of the Roman Catholic Church, dates back to the episcopate, whose functioning is modelled on the procedures of the Roman administration.

Shortly after his accession to the episcopate, an Eastern delegation of bishops led by Eusebius of Nicomedia requested Julius' communion for the Arian candidate for the episcopal see of Alexandria, against the Nicene Athanasius of Alexandria. The latter, who has regained his seat after having been removed from it by the Council of Tyre, in turn sends a delegation to support his point of view.

The bishop of Rome proposed conciliation but the Alexandrian bishop was again expelled from his seat by Constantius II and went into exile in Rome, accompanied by members of his clergy. Julius then gave his communion to Athanasius, alienating the Alexandrian delegation which refused to participate in a synod in Rome, considering that his bishop had posed as a judge of appeal, in defiance of the regionalization of ecclesiastical jurisdictions validated by the Council of Nicaea.

Julius argues that he merely continued the custom of establishing ecclesiastical unity "in the concrete manifestation of collegial brotherhood through the exchange of letters of communion" and the Roman synod, held in 340 or 341, annulled the election of Athanasius' competitor.

The co-emperors Constant and Constantius, under the repeated requests of the protagonists of the crisis, convoke in 343 a council in Sardia where the Eastern and Western delegations sit separately and excommunicate each other. The Westerners drew up a pro-Nicean symbol of faith that attempted to give a legal basis to the primacy of the Roman episcopal see, which remained, for the time being, without impact, while after the return of Athanasius to Alexandria in 346, the siege of Rome experienced a relative obliteration in the years that followed, not without Julius restoring Marcel d'Ancyre, accused of sabellianism, to his seat in 353.