You’re in luck - I did a presentation on church councils in my undergrad years! And even more important than me doing a presentation, is that fact that we have the complete verbatim transcript of the Acts of the Council of Chalcedon in 451 AD. I cannot stress enough how valuable this source is - the Roman Senate in all its one thousand years has not left behind a direct record of even a single session. The Acts were transcribed by both imperial scribes and other notaries working for important bishops. We have the Greek Acts as well as the Latin Acts of Chalcedon, but the official imperial Acts have disappeared into the sands of time.

How were all the attendees summoned? Well, by the emperor’s mandate. Constantine set a precedent when he took the initiative to call the Council of Arles in 314 for discussion of the Donatist Schism. From then on, all major church councils were called either with approval from the emperor or directly by the emperor himself. That is not to say bishops could not call church councils - provincial councils were often called by bishops to drum up support for a certain theological position, or to condemn a regional rival. However, an empire-wide council was another matter.

You see, and this is an oversimplification, there were four loci of power in the episcopical matters. One was in Constantinople, the capital of the empire. In most cases, the bishop of Constantinople had the same theological position and the most influence on the emperor. However, this was offset by the fact that the church in Constantinople was never a big player in church politics until the city became the empire’s new capital. Therefore, it could never draw upon the traditional powerbases or the esteemed theologians that the other major churches had. The sixth canon of Nicaea codified that the other major churches - Rome, Alexandria and Antioch - all enjoyed a special authority over churches in their regions based on historical precedent.The Antiochene church could often rally the whole of the Roman East to support its cause, while the Alexandrian church commanded Egypt and Libya. In Rome, successive Popes tried their best to stamp their authority on the church, but the bishops of the east took a dim view of their parroting and the Pope was merely considered a fellow church head, nothing more. The problem for the wider church was that at no point in time could Constantinople, Antioch, Alexandria and Rome all agree on the same theological position.

So say the Alexandrian bishop calls for a general church council to solve a doctrinal matter. The Antiochene church, which holds a fundamentally opposed position, now thinks: wait a minute, Rome supports Alexandria, Constantinople supports Alexandria, so...we’re outnumbered? What should we do? Let’s just boycott the whole thing and condemn it as heretical! And that’s what most church councils descended into, a refusal by the minority to attend. Enter the emperor, who has the appropriate God-given authority and right to call a general church council. Now the Antiochene bishop has to grudgingly join the imperially mandated council, along with his eastern following. The emperor’s summons was thus the essential prerequisite for an empire-wide church council.

The full imperial summons for the Council of Chalcedon in 451 AD:

Copy of the sacra sent by the most pious and Christ-loving emperor Marcian to the most God-beloved bishops everywhere concerning their all assembling at Nicaea.

The victors Valentinian and Marcian, glorious and triumphant, always Augusti, to Anatolius.

Before all matters the things of God should be given priority, for we are confident that, when almighty God is propitious, the commonwealth is both protected and bettered. Therefore, because certain doubts appear to have arisen about our orthodox religion, as is indeed shown by the letter of Leo, the most God-beloved bishop of the glorious city of Rome, this in particular has pleased our clemency that a holy council should be convened in the city of Nicaea in the province of Bithynia, in order that, when minds agree and the whole truth has been investigated, and after the cessation of those exertions with which some people have lately disturbed the holy and orthodox religion, our true faith may be recognized more clearly for all time, so that henceforth there can be no doubting or disagreement. Therefore your holiness should exert yourself to come to the aforesaid city of Nicaea by the Kalends of September with whatever most God-beloved bishops you choose and whomever from the churches in the care of your priesthood you consider to be trustworthy and equipped for the teaching of orthodox religion. May your God-belovedness know also that our divinity will attend the venerable council, unless perchance some public necessities engage us in a military expedition.

May God preserve you for many years, most holy and sacred father.

Issued on the tenth day before the Kalends of June in Constantinople in the consulship of our lord Marcian perpetual Augustus and the one to be designated.

This summons would be sent by imperial courier, along the highways of the Roman Empire to every major metropolitan bishop. The bishops then had the responsibility to inform lesser regional bishops of the summons. For Chalcedon, it was issued on ‘the tenth day before the Kalends of June,’ i.e. 23rd May, for the bishops to assemble ‘by the Kalends of September,’ i.e. 1st September. That gave ample time for the summons to be spread across the empire, and for the bishops to make suitable arrangements to arrive on time. Travel was coordinated - bishops in a faction arrived as a group to show solidarity. And bishops could be late, which often led to the opposing faction jumping the gun. At the Council of Serdica in 343 AD, the eastern bishops were late to arrive. To their outrage, they discovered the Council had already started and their hated enemy Athanasius had been admitted to communion by the western bishops. They promptly marched off to set up their own council. This incident almost led to a civil war between Constantius II and Constans, who supported the clergymen from their side of the empire.

Councils were split into sessions, one for each day. The Council of Chalcedon had sixteen sessions, but other councils could draw on for months if both sides were entrenched in their beliefs. Attendance lists were compiled at the start of every session. Councils regularly involved the condemnation of heretical beliefs of certain bishops, which drew vicious arguments between factions. Gregory of Nazianzus described his experience at the Council of Constantinople in 381 AD:

My speech was the signal for screams on every side from that flock of crows all massed together. The horde of young men, a new-fangled party, gave the impression of dust churned up by a whirlwind during a storm. In their confused chattering not even a ruler backed by reverential fear and authority could have managed to reason with them. They were like a swarm of wasps suddenly darting up in one’s face and, far from attempting to chasten them, the august assembly of the elders actually joined the demonstration.

At Chalcedon, the initial sessions were devoted towards the condemnation of Dioscorus, bishop of Alexandria, for presiding over the horrendously violent Council of Ephesus in 449 AD. The Council at Ephesus had descended into chaos after a thousand Alexandrian monks, brought over by Dioscorus, charged into the chamber and physically assaulted Flavian of Constantinople, who later died of his wounds.