A Muslim prays at Hagia Sophia mosque, in Iznik, also known by its ancient name Nicaea, northwestern Turkey, Thursday, Nov. 13, 2025, ahead of the visit of Pope Leo XIV to the city to mark the 1,700th anniversary of the First Council of Nicaea. (AP/Francisco Seco)
Centuries of church schisms show that if there's a doctrine to be fought over, there's a good chance Christians will fight about it.
That repeated splintering is what makes the Council of Nicaea — a meeting of bishops 1,700 years ago in present-day Turkey — so significant today. And why Pope Leo XIV is traveling on Nov. 28 to the site of this foundational moment in Christian unity as part of his first major foreign trip as pope.
In 325, the council hashed out the first version of the Nicene Creed, a statement of faith that millions of Christians still recite each Sunday.
"The occasion is very, very important — the first global, ecumenical council in history and the first form of creed acknowledged by all the Christians " said church historian Giovanni Maria Vian, coauthor of "La scommessa di Costantino," or "Constantine's Gamble," published in Italy in tandem with the anniversary.
Convened by the Roman emperor, Nicaea marked the first — but hardly the last — time that a powerful political leader took a leading role in shaping a far-reaching church policy. It was an early collaboration of church and state.
Leo will commemorate the 1,700th anniversary with Patriarch Bartholomew, the spiritual leader of Eastern Orthodox Christians.
Catholic, Orthodox and most historic Protestant groups accept the creed. Despite later schisms over doctrine and other factors, Nicaea remains a point of agreement — the most widely accepted creed in Christendom.
Other events have been commemorating the council, from the global to the local. The World Council of Churches, which includes Orthodox and Protestant groups, marked the anniversary in Egypt in October. At a Pittsburgh-area ecumenical celebration in November, the tongue-in-cheek catchphrase was, "Party like it's 325."
Unified empire, divided church
The Council of Nicaea is important both for what was done and how it was done.
It involved an unprecedented gathering of at least 250 bishops from around the Roman Empire. Emperor Constantine had consolidated control over the empire after years of civil war and political intrigues.
Constantine wouldn't formally convert to Christianity until the end of his life. But by 325, he had already been showing tolerance and favor toward a Christian sect that had emerged from the last great spasm of Roman persecution.
Constantine wanted a unified church to support his unified empire. But the church was tearing itself apart.
It's sometimes called the "Trinitarian Controversy," though the debate wasn't so much about whether there was a Trinity — God as Father, Son (Jesus) and Holy Spirit — but about how the Son was related to the Father.
Historians debate exactly who taught what, but an Egyptian priest named Arius gave his name to the influential doctrine of Arianism.
It depicted Jesus as the highest created being, but not equal to God. The opposing view, championed by an Egyptian bishop, said that Jesus was eternally equal to the Father.
An effort at compromise
Constantine called a council to sort things out. It's called the first "ecumenical" or universal council, as opposed to regional ones.
The bishops nearly unanimously supported a creed endorsed by the emperor. It's a shorter version of the Nicene Creed recited in church today. It declared Jesus to be "true God" and condemned those who proclaimed Arian ideas.
The creed described Jesus as equal to the Father, of "one substance" — "homoousios," a term from Greek philosophy rather than the Bible.
The council also adopted a formula for determining the date of Easter, which had been controversial. The council approved the calendar favored by Arian sympathizers, setting Easter for the Sunday after the first full moon of spring. That gave each side a win, said David Potter, author of "Constantine the Emperor" and a professor of Greek and Roman history at the University of Michigan.
"The Council of Nicaea was an extraordinary diplomatic success for Constantine, because he got the two sides to agree," he said.
As a result, an emperor's theological legacy endures.
"I've often thought that it's nice that a piece of imperial legislation is read out every Sunday," Potter said.
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Ominous language about Jews
When the council set its formula for determining Easter, it made a point of distancing the observance from that of Jewish Passover. It used highly contemptuous language for Jews.
"Institutional antisemitism was absolutely a feature of the church," Potter said.
He noted that such harsh language was common on all sides of ancient religious disputes among early Christians, Jews and pagans. But it helped set a precedent for centuries of persecution of Jewish minorities in Christian lands.
The settlement unsettled
Despite agreement on the creed, it didn't settle things. In fact, Arius made a comeback, returning to political favor.
Doctrinal debate raged for another couple of generations — even in the streets of the new capital of Constantinople.
"Old-clothes men, money changers, food sellers, they are all busy arguing," wrote St. Gregory of Nyssa late in the fourth century. "If you ask someone to give you change, he philosophizes about the Begotten and the Unbegotten. If you inquire about the price of a loaf, you are told … the Father is greater and the Son inferior."
In 381, another emperor convened a council in Constantinople. It affirmed an expanded Nicene Creed, with added lines describing the church and the Holy Spirit. The final version became the standard text used today. It's sometimes called the Nicene-Constantinopolitan Creed.
Later -isms and schisms
That largely took care of the Arians, but new controversies arose in later centuries.
Some churches in Asia and Africa, including the Oriental Orthodox bodies, accepted the Nicene Creed but rejected later councils amid disputes over how to talk about Jesus being both human and divine. Pope Leo, while in Turkey, also plans to meet with representatives of two Oriental Orthodox groups, the Armenian Apostolic and Syriac Orthodox churches.
The Catholic and Eastern Orthodox churches had their own schism in the 11th century. They'd already been growing apart over such things as papal authority, but a big controversy was that the Western churches had added a clause in the Nicene Creed that the Eastern ones hadn't agreed to. Specifically, the original creed said the Holy Spirit "proceeds from the Father," but Catholics added, "and the Son."
Protestant churches later split over other issues, though most held to the Nicene Creed. Historic churches such as Lutherans, Anglicans and Presbyterians explicitly affirm the creed. Many modern evangelical churches that don't officially affirm the creed, such as many Baptists, have their own statements of faith that largely agree with it.
A few notable exceptions, such as Jehovah's Witnesses and The Church of Jesus Christ of Latter-day Saints, don't accept the Nicene formula.
The Catholic and Protestant churches also began observing Easter differently than the Orthodox a few centuries ago, using an updated solar calendar — and opening yet another breach in Nicene unity.
Still, Nicaea offers hope to a divided church, said the Rev. John Burgess, a systematic theology professor at Pittsburgh Theological Seminary who is a Presbyterian minister and a scholar on Eastern Orthodoxy.
"An event like the 1,700 years of Nicaea is really the celebration not of a reality but of a hope — of what Christians at their best know ought to be the case, that there is a deep call to unity," he said.
