Council of Serdica

The Council of Serdica, or Synod of Serdica, was a synod convened in 343 at Serdica in the civil diocese of Dacia, by Emperors Constans I, Augustus in the West, and Constantius II, Augustus in the East. It attempted to resolve "the tension between East and West in the Church." "The council was a disaster: the two sides, one from the west and the other from the east, never met as one."

Calling of the Council

"Constans decided to take the initiative... His brother Constantius … agreed to permit, at the suggestion of Constans, that a grand Ecumenical Council should take place, with the intention of resolving the tension between East and West in the Church, at Serdica, modern Sofia, a city carefully chosen as standing between the Eastern and Western halves of the Roman Empire."
Traditionally, it had been claimed that the council was convened by the two augusti at the request of Pope Julius I. However, RPC Hanson wrote:

"The devisers of this meeting were certainly not Eastern bishops. Socrates expressly says that they did not want to come. It was a small group of Western bishops, influential with Constans, who planned the Council: Maximinus of Trier, Protasius of Milan, Ossius of Cordova, Fortunatianus of Aquileia and Vincent of Capua. Julius of Rome was not a prime mover in the affair; he sent a comparatively minor delegation who kept a low profile."

Since the "devisers" of the council were a small group of bishops who had Constans' ear, the question arises to what extent they represent the general view of the West. After discussing the evidence, Ayres concludes that it is not accurate to describe it as an East/West or a Latin/Greek divide. He says it is an error to assume "that Greek-speaking areas of the east divided clearly in theology from the Latin-speaking west. … ‘East’ vs. ‘West’ is far too clumsy a tool of analysis for almost anything in the fourth century."

Delegates

"In 343 … about 90 bishops from the West and about 80 from the East set off to meet in Serdica. Constans himself, accompanied by Athanasius and several other Eastern bishops who had been deposed during the past twenty years, attended the encounter." "Athanasius, Asclepas and Marcellus were present as Eastern bishops with a grievance." These Eastern bishops were deposed by Eastern courts. Their attendance under the protection of the Western emperor was a direct challenge and insult by Emperor Constans to the authority of the Eastern church.
At this time, "Constantius was on the Eastern frontier occupied with war against the Persians." He did not attend. "He sent the comes Strategius Musonianus and the castrensis Hesychius, to assist the Eastern bishops in their journey and ordered Philagrius, to accompany them on their journey from Philippopolis to Serdica."
Consistent with the principle that a small group of Western bishops were the "devisers" of this council, the Western delegates came from a relatively small part of the Western Empire:

"At least half of those attending the ‘western’ meeting were from areas to the east of northern Italy and the largest single block of attendees were the Greek and Balkan bishops. The ‘western’ council was as localized as most during this century."

Julius I was represented by the priests Archidamus and Philoxenus, and the deacon Leo. Athanasius reported that bishops attended from Roman diocese of Hispania, Gaul, Britain, Italy, Africa, Egypt, Syria, Thrace and Pannonia. Constantius II was represented by Strategius Musonianus and Hesychius of Antioch.

The Council Never Met

Later descriptions claim that the event was never a council.
"The council was a disaster: the two sides, one from the west and the other from the east, never met as one." "It was in fact a debacle rather than a Council, and it is absurd to reckon it among the General Councils."
"The unwilling Eastern bishops... on reaching Serdica were housed in a wing of the imperial palace and carefully kept from informal contact with the Western bishops." They refused to allow Marcellus and Athanasius to participate in the discussions and the Westerners would not allow the meeting to continue without them:

"The majority refused to meet with the ‘westerners’ who wished Athanasius and Marcellus to be allowed normal participation in the meeting." These two bishops "had been tried, condemned and deposed by regularly convened and ordered Eastern councils." Athanasius had been found guilty of "tyrannical behaviour." "The Easterners had no intention of allowing the Westerners to review decisions which they were competent to make. … The Easterners had a perfectly good case, and this fact until recently has not been sufficiently realized. Western bishops had no right to review the verdicts of Eastern councils. … Metropolitan jurisdictions were fairly clearly established in the East but were still in an uncertain and unformed state in the West."

"Athanasius was deservedly unpopular in the East. Serious attempts were made to overcome the impasse. Ossius more than ten years later said that he had gone so far as to offer to take back Athanasius with him to Spain if the Easterners would join him in discussion, but the distrustful Eastern bishops refused this suggestion."
Both sides took the most imprudent measures towards the others: "The Western bishops examined the cases of Athanasius, of Marcellus, of Asclepas and of Lucius all over again and declared them innocent." They "stigmatized all the Easterners as Arians" and excommunicated Eastern leaders. "The ‘easterners’... excommunicated all the ‘western’ leaders at Serdica" and "branded all the Westerners as Sabellians." "Intended as a means of healing a dangerous rift which was developing between the Eastern and Western Church, it succeeded only in widening that rift to an apparently unbridgeable extent."

Historical context

Council of Nicaea

This dispute that prevented the entire council from meeting already began at Nicaea, where Alexander formed an alliance with Marcellus and some other Sabellians:

"Simonetti estimates the Nicene Council as a temporary alliance for the defeat of Arianism between the tradition of Alexandria led by Alexander and 'Asiatic' circles whose thought was at the opposite pole to that of Arius. … Alexander … accepted virtual Sabellianism in order to ensure the defeat of Arianism." "Marcellus learnt the main lines of his theology from Eustathius." "Eustathius and Marcellus … certainly met at Nicaea. and no doubt were there able to join forces with Alexander of Alexandria and Ossius."

Since Constantine had taken Alexander's part, Alexander’s alliance was able to dominate the council, including to insert in the Creed the term homoousios which hitherto was preferred only by the Sabellians.

"Ossius of Cordoba probably chaired the meeting; Eustathius of Antioch, Marcellus of Ancyra, and Alexander must all have been key players in the discussions." "Marcellus of Ancyra … had been an important figure at the council and may have significantly influenced its wording."

"Athanasius was certainly present as a deacon accompanying Alexander of Alexandria. … But it is equally certain that he can have taken no prominent nor active part, in spite of later legends to this effect and the conviction of some scholars that he was the moving spirit in the Council."

After Nicaea

After Nicaea, Marcellus was deposed for Sabellianism. "Marcellus of Ancyra had produced a theology … which could quite properly be called Sabellian."
After Alexander died in 328, Athanasius, who was still underage, became bishop of Alexandria. However, more or less at the same time as Marcellus, Athanasius was found guilty of violence and "tyrannical behaviour" and exiled. Both Marcellus and Athanasius were bishops in the eastern part of the Empire, were deposed by the Eastern Church, and were exiled to Rome. What is less well known, is that both Athanasius and Marcellus taught one single hypostasis.

"Athanasius and Marcellus could come together in Rome. The perception that these two trajectories held to very similar beliefs would help to shape widespread eastern antipathy to both in the years after Nicaea." "The fragments of Eustathius that survive present a doctrine that is close to Marcellus, and to Alexander and Athanasius. Eustathius insists there is only one hypostasis."

For that reason, Athanasius and Marcellus, while in Rome, were able to form an alliance against those who taught that the Father, Son, and Spirit are three hypostases :

"They considered themselves allies." "Athanasius and Marcellus now seem to have made common cause against those who insisted on distinct hypostases in God."

While in Rome, Athanasius also developed his polemical strategy:

"Athanasius’ engagement with Marcellus in Rome seems to have encouraged Athanasius towards the development of" "an increasingly sophisticated account of his enemies;" "the full flowering of a polemical strategy that was to shape accounts of the fourth century for over 1,500 years;" "a masterpiece of the rhetorical art."

This included the claim that all of his enemies are Arians, which they were not, and that Athanasius himself was deposed due to an Arian ‘conspiracy’, which is also not true.
Constantine was emperor of the entire Roman Empire and was able to limit religious disagreements between factions in the church. However, after he died in 337, his sons divided the empire between them. This created the opportunity for theologies to develop in different directions in the eastern and western parts of the empire.
After the empire was divided, Athanasius was able to convince the bishop of Rome of his polemical strategy. "Athanasius appealed to Julius of Rome in 339–40 by using his strategy of narrating a theological conspiracy of ‘Arians’. His success had a profound impact on the next few years of the controversy." He and Marcellus were also able to convince the bishop of Rome of their orthodoxy and of Athanasius’ innocence. "Julius, in the year 341, summoned a council to Rome, which vindicated the orthodoxy of Marcellus, as well as that of Athanasius." However, since both Marcellus and Athanasius were Eastern bishops and were deposed by the Eastern Church, their vindication by the Western Church created tension between the East and the West. In the year 341, the bishop of Rome attacked the Eastern Church by means of a letter, using Athanasius’ polemical strategy and accusing the Eastern Church of being ‘Arians’, meaning followers of Arius. "Julius wrote to the east in 341 in a letter which shows the strong influence of the emerging Athanasian account of ‘Arianism’." This exacerbated the division between East and West. "Once Julius had acted we begin to see divisions between the Church in the eastern and western halves of the empire emerging." The Eastern Dedication Council of 431 discussed and rejected that letter.
"Early in the year 342 a delegation from the Eastern Church presented itself at the court of the Emperor Constans in Trier. … It carried with it the Fourth Creed of Antioch 341 and asked the Emperor to consider it. As a gesture of reconciliation, this embassy was fruitless, because nobody in the West took any notice of the creed."