In 325 AD – exactly 1700 years ago to this date – the First Council of Nicaea brought bishops together under imperial invitation, not divine calling. They didn’t meet to settle all disputes, but their gathering marked the beginning of what would become a long tradition of theological councils.

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These days, we are marking the 1700th anniversary of the Council of Nicaea. Recently, I had the opportunity to visit the Vatican once again, and a few months later, I travelled to Istanbul. Many of the photos are still waiting to be edited, but this anniversary seems like the perfect moment to begin a series of posts on the history of the Byzantine Empire and the early days of Christianity. What adds to that is that in fact, today we have a new pope for a few days now, and from what I’ve observed, posts related to the papacy and the Vatican are generating a lot of interest.

The Council of Nicaea, more precisely the First Council of Nicaea, was the first Christian council, convened in 325 AD by the Roman Emperor Constantine the Great. It took place in late May and early June in Nicaea (modern-day İznik in Turkey, in Asia Minor – and should not be confused with Nice in France) and brought together around 300 bishops, mainly from the eastern part of the Roman Empire. At that time, the Eastern part of the empire was significantly more Christianised. It was there that Christianity had developed earliest and most dynamically. As well, the main theological controversy that led to the council’s convening – the Arian dispute – also originated in the East. In fact,

Emperor Constantine himself was, for economic and political reasons, residing in Asia Minor at the time, with his court and administrative base located in the eastern part of the empire. This facilitated communication with the local bishops and the organisation of their participation.

Constantine the Great, also known as Caesar Flavius Valerius Constantinus, was a Roman Emperor who reigned from AD 306 to 337. He was the first emperor to embrace Christianity (although he was baptised only on his deathbed) and played a pivotal role in its legalisation and spread throughout the Roman Empire. His reign marks a turning point in the history of Christianity and the transformation of the Roman Empire. In 313 AD, Constantine issued the Edict of Milan, which guaranteed freedom of religion and brought an end to the persecution of Christians. His support for the Church included, among other things, funding the construction of churches (such as the Old St. Peter’s Basilica in Rome – not to be confused with the current one), exempting clergy from taxes, and intervening in doctrinal disputes – most notably by convening the Council of Nicaea. Constantine is also renowned as the founder of the new imperial capital – Constantinople (modern-day Istanbul), which he established in 330 AD on the site of the Greek city of Byzantion. Although Constantinople was never officially declared the new capital in a legal sense, it quickly assumed Rome’s role in practice. The city gradually became the new administrative, military, and religious centre of the Roman Empire, and its strategic location on the Bosphorus made it superior to the old Rome, which was increasingly vulnerable to barbarian invasions. When the Western Roman Empire fell in 476 AD, Constantinople became the sole capital of the Roman Empire, which from then on existed solely as the Eastern Empire, now known as the Byzantine Empire. This empire endured for nearly another thousand years, until it was conquered by the Ottoman Turks in 1453.

Let us return, however, to the topic of the Council of Nicaea. This council was of great significance in shaping the foundations of Christian doctrine and marked a milestone in Church history, as it was the first in a series of many councils that have taken place, with varying regularity, ever since.

Importantly, this was a council held at a time when the papacy did not yet formally exist. The institution of the Bishop of Rome was already in place, but he was not yet officially recognised as the head of the entire Church – only of the Roman Church. As mentioned earlier, the Eastern Church was more developed at that time, and the Bishop of Rome did not attend the council in person, instead sending his legates (representatives).

The main purpose of the Council was to settle the Arian controversy. Arius was a Christian presbyter (priest) from Alexandria in Egypt, who lived around the turn of the 3rd and 4th centuries (c. 256–336 AD). He became known as the founder of the doctrine now referred to as Arianism. Arius taught that Jesus Christ, as the Son of God, was not of the same substance as the Father, but had been created by Him. He famously said, “There was a time when the Son did not exist,” implying that Christ was not eternal and did not share the same divinity as God the Father. This concept undermined a core tenet of Christian doctrine – the Holy Trinity, which holds that the Father, Son, and Holy Spirit are equal and co-eternal. Arius’s views quickly gained support among some clergy, particularly in the East, leading to a sharp conflict within the Church.

At the time the council was convened, Arius had already been condemned by his bishop, Alexander of Alexandria, and had been formally excommunicated. Nevertheless, his teachings had gained influential supporters, who defended his doctrine during the proceedings. Arius himself was present in Nicaea, though not as a full participant in the bishops’ discussions. He was treated more like an accused heretic. Following the deliberations, he was once again condemned, and his teachings were declared heretical. Arius was sent into exile, and his writings were ordered to be burned. As a result of the council’s deliberations, the first version of the Creed – the profession of faith – was formulated. A profession of faith is a concise, official declaration of the core beliefs held by Christians. However, this Creed was later expanded during subsequent councils. The Nicene Creed became the foundation of the profession of faith for the Catholic Church, the Orthodox Church, and many Protestant communities.

What might the Council of Nicaea have looked like? I asked AI to generate an image of the event, keeping in mind that it took place nearly two thousand years ago – so whether it truly looked like this remains uncertain. I specifically requested a departure from both traditional iconographic representations and the anachronistic Renaissance portrayals, which often distort the historical realities of the 4th century. Therefore, the image shows no golden halos, no glorified bishops, and no titles or insignia that did not exist at the time. At the centre sits Emperor Constantine the Great – in his prime, clean-shaven, dressed in a purple imperial robe and wearing a diadem, the symbol of imperial power and divine authority in the Roman Empire. Around him, a theological debate takes place among bishops, dressed in simple early Christian liturgical garments – with no gold embroidery or mitres. This clothing reflects the true fashion of the 4th century, not the Middle Ages. The Bishop of Rome was not present at the council, as he did not yet serve as “pope” in the sense known from later centuries – he held no superior jurisdiction over other bishops. Only his legates took part in the council.

One of the important decisions of the council was the unification of the date for celebrating Easter. Some Christian communities still observed it in parallel with the Jewish Passover (on the 14th day of the month of Nisan), regardless of the day of the week – a practice that caused controversy. The council decided that Easter should be celebrated on a Sunday, following the spring full moon, but not at the same time as the Jewish Passover. This gave the celebration of the Resurrection of Christ a unified and distinctly Christian character. The council also introduced the first disciplinary canons – rules governing the organisation of church life. These included, among others: procedures for readmitting heretics and apostates to the Church; the structure of ecclesiastical hierarchy, confirming the primacy of metropolitans and the privileges of bishops of apostolic sees (Rome, Alexandria, Antioch); a ban on clergy transferring between dioceses without the bishop’s consent; and disciplinary measures against clergy for inappropriate conduct (such as abandoning celibacy or disobedience).

Today, a council in the Christian Church is a formal gathering of bishops, convened to make decisions on the most important matters of faith, liturgy, church discipline, or church governance. Councils may be local, involving only representatives from a given region, or universal – known as ecumenical councils – which represent the entire Church and issue decisions regarded as binding for all the faithful. In the Christian tradition, particularly in the Catholic Church, councils play a major role in shaping Church doctrine and identity. To date, the Catholic Church recognises 21 ecumenical councils. The Council of Nicaea was the first of them. In the centuries that followed, councils were convened in Constantinople, Ephesus, Chalcedon, and again in Nicaea, which laid the foundations of Christology and the doctrine of the Holy Trinity. These first seven councils are also recognised by the Orthodox Church. Later councils, such as the Fourth Lateran Council (1215), the Council of Trent (1545–1563), Vatican I (1869–1870), and Vatican II (1962–1965), were pivotal in shaping modern Catholicism.