Arles and Milan
Sozomen, HE IV.8 gives an account of the council. He mentions over 300 Western bishops, few Eastern bishops. The Easterns insisted that Athanasius be condemned to banishment. Sozomen records only Dionysius of Alba, metropolis of Italy, Eusebius of Vercella, Paulinus of Treves, Rhodan(i)us (of Toulouse) and Lucifer, as bishops who opposed this decision. They protested that it was a “plot” to suppress Nicene faith, and they were banished. Sozomen then notes that Hilary was exiled with them. Sozomen seems dependent upon Rufinus for his account.
Socrates gives a brief account in HE ii.36. He agrees with Sozomen in listing Dionysius bishop of Alba, metropolis of Italy; whereas Athanasius in Epist. ad. Solitar has him as bishop of Milan, metropolis of Italy. The Synod at Ariminum that follows is seen by both Sozomen and Socrates as a result of Milan, and Constantius’ desire to achieve ecumenical unity on the issue.
Theodoret HE ii.12 also treats of the Council of Milan. He adds little new, and depends chiefly upon Athanasius.
Rufinus HE i.19,20 also discusses the Council, but without much further detail.
Sulpitius Severus in Historia Sacra ii.39 mentions not only the Milan council, but the Arles and Bitterae [sic] councils. He characterises the stand-off as the Eastern/Imperial attempt to secure condemnation of Athanasius, matched by Western determination to first discuss the true faith, and then consider the question of Athanasius. He refers to Paulinus’ banishment as prior to Milan, while Dionysius subscribed to the condemnation of Athanasius, and was only banished afterwards when he refused to concur with a letter of Valens and Ursacius. Furthermore, the case of Rhodanius’ exile is given as resistance based upon “fellowship with Hilarius”. Only Severus gives us this indication that Hilary had any leading role at all.
Athanasius in Hist.Arian 31-46 gives an account of the period, mention both Arles and Milan. He depicts the events as driven by Constantius and his preference for ‘Arian’ doctrines. However Athanasius is light on details of the events of those councils, instead focusing on Imperial measures aimed at securing his condemnation, and the heroic confession of those exiled at Milan, named above. Athanasius makes no mention of Hilary here.
It has been typical to date Hilary’s exile to the 356 council of Baeterra. The reason for this is disputed. The traditional reading has understood it as the result of Hilary’s stance for Nicene Orthodoxy. Alternative views have been that it was a charge of immorality (H.M. Gwatkin), of political (Feder, Chadwick, Christof). Our primary sources for understanding this council are Hilary himself, in Ad Constantium. Coupled with Hilary’s de Synodis, and the omission of Hilary in Athanasius’ account of bishops deposed for resisting Arianism. The text is dateable to 360, since he makes claims about Constantius’ legitimacy as emperor, which he would almost certainly not do if he knew of the proclamation of Julian. He further indicates a period of five years (quinto abhinc anno) from the events that he briefly narrates: the exiles of Paulinus, Eusebius, Lucifer, Dionysius (the later three occurring at Milan in 355, Paulinus at Arles), an act of rupturing communion from Saturninus, Ursacus and Valens but together with Gallic bishops, then being compelled to attend the council at Baeterra (ad Biterrensem synodum compulsus), where he attempted to present a denouncement of their heresies, but was prevented, and from then exiled.
Barnes treatment continues with an examination of chapter 11 of Ad Constantium. In this section Constantius is compared to a ‘rapacious wolf’. He argues particularly (i) that Constantius is compared to ‘a Nero’, i.e. a pagan persecutor, but is worse because one cannot flee a so-called ‘Christian’ persecutor like Constantius. (ii) reference to Paulinus’ exile, couple with ‘edicts that terrorised the faith’ – to be identified with the push for Arles to ratify Sirmium 351. Further reference to Paulinus being twice-exiled, first to Phrygia and then extra christianum nomen. (iii) a clear reference to the arrest of Dionysius of Milan in 355; (iv) Liberius’ exile and return, which situates the arrest between Milan and Baeterra (so autumn 355), and a return in 357. (v) that Rhodanius was expelled after the council of Baeterra; Jerome says that he was exiled at Arles, Rufinus and Sozomen in conjunction with Milan, but those are almost certainly simple conflations of 3 councils held in quick succession.
Barnes’ treatment then continues with the examination of the opening letter of De Synodis particularly with the inclusion of et provinciae Aquitanicae; this would mean that Hilary greets the bishops of all provinces except Sequania, Alpes Graiae et Poeninae and Alpes Maritimae, Viennensis, and Narbonensis, but conspicuously greets the province as a whole and the clergy of Toulouse. Barnes conjectures that it is because the council was a provincial synod of Narbonensis, that the Narbonensene bishops deposed him, and that the council was held between 356 April 7th and May 17th.
The last part of Barnes’ article deals with the question of why Hilary was exiled; Athanasius understandably omits Hilary, because Hilary in his de Synodis gives some praise for Antioch 341 and Sirmium 351, and these two councils condemn Athanasius. Athanasius has no desire to align himself with someone who endorses councils that condemn him. This might imply that those councils were valid and so their condemnation of Athanasius was valid. The marginal notes of Hilary in the copy of De Synodis sent to Lucifer of Cagliari seem to indicate a defense by Hilary of himself, that Hilary tore up the document that was brought to him from Milan – documents agreeing with the condemnation of Lucifer and others at Milan, and affirming the Creed presented there.
 ibid., p130
 ibid., p132-4.
 ibid., p135.