Nicaea / Iznik Pilgrimage Guide:
The First Ecumenical Council & Pope Leo XIV's Historic Visit

In June 325 AD, in a lakeside city on the edge of the Sea of Marmara, Emperor Constantine I gathered the bishops of the Christian world to resolve the greatest theological crisis the young church had faced. Approximately 318 bishops — some still bearing the scars of the Diocletianic persecutions — assembled in Constantine's palace at Nicaea to answer a single, earth-shaking question: was Jesus Christ truly God, or a supremely exalted creature? Their answer, hammered out in debate over two months, became the Nicene Creed — recited by an estimated two billion Christians in worship every Sunday to this day.

The city where it happened is now Iznik, a quiet Turkish market town on the shore of Lake Iznik in the province of Bursa. Its Roman street grid, Byzantine walls, and submerged basilica make it one of the most historically significant — and least-visited — Christian pilgrimage sites in the world.

On 28 November 2025, Pope Leo XIV and Ecumenical Patriarch Bartholomew I stood together at the lakeshore of Iznik to mark the 1,700th anniversary of the First Council of Nicaea. Their joint presence — the Bishop of Rome and the first bishop of Orthodoxy, praying together at the place where their shared creed was born — was one of the most resonant moments of ecumenical Christianity in the early 21st century.

The First Council of Nicaea, 325 AD

The council was called by Constantine — who had unified the empire after years of civil war — partly for theological reasons and partly for political ones. A fractured church, arguing bitterly about whether the Son was equal to the Father, was an embarrassment to an emperor who saw Christianity as a force for imperial unity. The dispute had begun in Alexandria, where a popular and persuasive priest named Arius was teaching that the Son of God, however exalted, had been created by the Father and was therefore subordinate to him: "there was a time when he was not."

The council's response was to insert a single Greek word into the creed: homoousios — "of the same substance." The Son is not like the Father, not subordinate to the Father, not a secondary divine being: he is consubstantial with the Father. This was the theological line in the sand, and it held. The council also settled the date of Easter and addressed dozens of questions of church discipline. Its canons shaped Christian canon law for centuries.

The great champion of Nicene orthodoxy was the Alexandrian deacon Athanasius (later bishop), who spent much of the rest of his life exiled for defending the council's conclusions against emperors sympathetic to Arianism. His tenacity gave rise to the phrase still used in English: "Athanasius contra mundum" — Athanasius against the world. His feast day is 2 May in the Western church and 18 January in the East.

The Seventh Council, 787 AD: Nicaea and the Icons

Four and a half centuries after the First Council, Nicaea hosted another ecumenical gathering — the Seventh Ecumenical Council of 787 AD. The question this time was not about the nature of Christ but about how he could be depicted. The Iconoclast controversy had torn the church and the Byzantine Empire for sixty years: emperors had ordered the destruction of icons, arguing that depicting Christ, the Virgin, and the saints in images was a form of idolatry. Monks, nuns, and ordinary faithful had resisted, often at the cost of their lives or limbs.

The council, convened under Empress Irene, restored the veneration of icons, carefully distinguishing between proskynesis (veneration, offered to icons) and latreia (worship, offered to God alone). The icon controversy had a further consequence: it widened the gulf between Rome and Constantinople, contributing to the trajectory that would eventually produce the Great Schism of 1054. The site associated with the Seventh Council is the Hagia Sophia of Iznik — today the Orhan Camii mosque, in the heart of the modern town.

The Underwater Basilica: Nicaea Rediscovered

In 2014, researchers from Uludağ University in Bursa were studying aerial photographs of Lake Iznik when they noticed something unexpected: beneath the shallow eastern waters, the unmistakable rectangular outline of a large basilica. Subsequent underwater investigation confirmed a 4th–5th century Christian church, dedicated to Saint Neophytos, a local martyr of the Diocletianic persecutions. The basilica is submerged because the lake level has risen by approximately 2 metres since late antiquity.

The discovery was significant for more than archaeological reasons. The basilica sits very close to the area where Constantine's imperial palace is believed to have stood — the building where the 325 council actually convened. The possibility that the church was built directly over or beside the council's meeting place gives the site an extraordinary layered significance: a martyr's basilica, possibly built to commemorate the very ground where the Nicene Creed was written. A viewing platform on the lakeshore now allows visitors to see the basilica's outline on calm, clear days.