Council of Sardica

The Council of Serdica, or Synod of Serdica, was a synod convened in 343 at Serdica in the civil diocese of Dacia, by Roman dominate Emperors Constans I, augustus in the West, and Constantius II, augustus in the East. It attempted to resolve the Arian controversy, and was attended by about 170 bishops. It was convened by the two augusti at the request of Pope Julius I.

T. Gilmartin, (Professor of History, Maynooth, 1890), writes in Church History, Vol. 1, Ch XVI: This most important Council of Sardica, Sofia, was held for the purpose of inquiring into the charges alleged against Athanasius of Alexandria and other bishops, on account of which they were deposed from their sees by the Semi-Arian Synod of Antioch (341) and sent into exile. It was convoked, according to Socrates, (E. H.., ii. 20.), by the two Emperors; Constans and Constantius; but according to Baronius, by Pope Julius (337-352). If it was convoked by the Emperors, but it was with the sanction and authority of the Pope. There were present 176 bishops from the East and the West. Hosius of Corduba, who presided at the Council of Nice, presided at this. &

The first ecumenical council (Nicaea I) canon 5 decreed that bishops shall convene in biannual synods within every province to act as a court of second instance and review cases with excommunication sentences pronounced by individual bishops. But, there was no appeal to a court of final instance "if an unjust sentence was imposed" by a provincial synod acting as a court of second instance. Nicaea I canon 5 "implied that" provincial synods "had an acknowledged authority to" judge the acts of individual bishops of their province. Provincial synods' authority "was becoming well established in the East" prior to the council of Serdica. In 431, the Synod of Antioch canons 14 and 15 "were designed both to augment the authority of the provincial synod as a trial court and to ensure the integrity of its operation."

...
Wikipedia