In 325 AD, Emperor Constantine invited every bishop in the church to gather in Nicaea and formally establish Christian doctrine. The goal was to unite the increasingly divided church with a set of beliefs its leaders agreed on and would hold each other accountable to.

This meeting, known as the First Council of Nicaea, was specifically called to make a decision about Arianism—the belief that God created Jesus, and that Jesus was not eternal or one with God. For the first time, leaders from every corner of the church would formally declare who Jesus was in relation to God.

Arianism was growing in popularity, even among church leaders, and those who opposed it believed salvation was at stake—if Christians were wrong about who Christ was, did they really even believe in him?

Emperor Licinius (who was emperor until 324 AD) thought the dispute was meaningless. But by 325 AD, these two competing ideas of who Jesus was were threatening to tear the church—and by extension, the newly Christianized Roman empire—in two.

Constantine wasn’t necessarily interested in the theological outcome, so long as it put an end to the division. So he called together the church’s first ecumenical council—a gathering of leaders from the global church.

Some have argued that the First Council of Nicaea invented the Trinity, and that its statement of faith didn’t truly reflect the beliefs of the church. Every bishop was invited, but only a fraction (traditionally 318) of them showed up at the council. So how could the council’s decisions really represent the entire church, especially at a time when the church was so divided? And since council members had to sign the statement of faith or face excommunication, were they really in agreement, or simply saving their skin?

Not to mention, the church continued to debate (and even overturn) the council’s decision in the centuries that followed.

The First Council of Nicaea played a pivotal role in the early church, and directly impacted the doctrine Christian churches uphold today. The council produced the Nicene Creed, which many churches around the world still use as their statement of faith.

But what exactly was decided at this council, and who decided it?

Here’s what we know about the Council of Nicaea.

Why was the council needed?

Constantine called the council to make a decision about Arianism. But Arianism had only inflamed divisions that began long before. For years, the church had disputed the nature of Christ and struggled to agree on his relationship to God.

The story of the Council of Nicaea is bigger than Arianism.

Are Jesus and God one person, or two?

Nearly a century before Arianism emerged, the church made a decision about another heretical belief related to Christ’s identity: Sabellianism. Named after Sabellius, the priest who primarily advocated for this position, Sabellianism was the belief that while Jesus was divine, he was essentially a manifestation of God and not a distinct being. God the Father, Jesus the Son, and the Holy Spirit were three “aspects” of one being: God.

None of Sabellius’ writings have survived, so all we know about his teachings comes from his critics, who deemed him a heretic. Around this time, the church was also grappling with a very similar heresy: modalism.

Many popular analogies that people use to describe the Trinity could technically be described as modalism. People often say the Trinity is like water, steam, and ice: three different forms of the same thing. But unless you specify that you mean three separate bodies of water, it’s modalism. God the Father, Jesus, and the Holy Spirit aren’t just three separate forms of one being, they are distinct persons, but one in nature.

If you’ve ever looked into what the Bible says about the Trinity or tried to explain it to someone, you know this concept can still create confusion today, so it’s no surprise that it took the church a long time to agree on it. (And we’re still not 100% on the same page—if you don’t believe me, read the comments on any blog post about the Trinity.)

By the fourth century, the church had largely agreed that these beliefs confused the nature of Christ and improperly defined his relationship to God.

But then a priest named Arius came onto the scene.

“There was a time when the Son was not”

In the early fourth century, Bishop Peter of Alexandria excommunicated Arius for his associations with a bishop named Meletius, who may have even ordained Arius as a priest. (This association will come up again later.) But Peter’s successor, Achillas, reinstated Arius, and within two years of being excommunicated, he was already a church leader again—this time in the oldest church in Alexandria.

Socrates of Constantinople, an early church historian, tells us that “the Arian controversy” began when Arius heard Alexander of Alexandria (who had become bishop of Alexandria after Achillas) give a sermon about the Trinity’s unity.

Arius believed this sermon implied that Jesus and God the Father were two aspects of one being—a resurgence of Sabellianism. So he argued, “if the Father begat the Son, he that was begotten had a beginning of existence: and from this it is evident, that there was a time when the Son was not. It therefore necessarily follows, that he [the Son] had his substance from nothing.”

The councils before the Council of Nicaea

Arius’ teachings spread and gained the support of numerous church leaders. Alexander called two separate meetings for his priests to decide what to do about Arianism, but they couldn’t reach a consensus, and meanwhile, Arius’ reputation continued to grow.

Five years before the First Council of Nicaea, in 320 AD, Alexander called a much larger meeting, a synod of the entire church of Alexandria and the neighboring church of Maerotis. 80 church leaders—including Athanasius of Alexandria, who would later succeed Alexander—signed a document declaring Arianism heretical.

So Arius went and spread his teachings somewhere else. And he continued to gain traction, even winning over two bishops.

Arianism had already fractured the church, but now it was on the brink of splitting it in two.

By 321 AD, Alexander was getting desperate, and he called together a council of the entire Roman church. (Not the Council of Nicaea.) More than 100 church leaders showed up, and Arius made his case, doubling down on his previous position but adding that the Son was not the same essence as the Father—which is an awful lot like saying Jesus wasn’t God.

The council was shocked by the addition, and they excommunicated Arius again.

The emperor intervenes

Arius began spreading his teachings in Palestine, where more bishops rallied behind him, and he continued to face opposition from Alexander and others. Arianism became such a prominent issue that Emperor Licinius I (the emperor before Constantine) wrote to Alexander and Arius, asking them to play nice.

It didn’t work.

And some Arians began using violence to defend their beliefs.

Alexander wrote to all the bishops, giving his account of Arianism and its flaws. Meanwhile, Eusebius of Nicomedia (not to be confused with Eusebius of Caesarea), held a separate council to review Arius and the actions taken against him.

And they readmitted him to the church.

Alexander wrote a statement of faith and got more than 250 church leaders to sign it.

Constantine (now the emperor) wrote to Alexander and Arius, again, asking if everyone could please just get along.

So Alexander summoned another council (still not the Council of Nicaea), which agreed with his statement of faith and said that Arius was still excommunicated. Oh, and that the followers of Meletius (the guy who Arius was first excommunicated for supporting) weren’t really part of the church.

Arius was pretty unhappy, so he complained directly to Constantine.

So Constantine invited Arius to make his case in front of the entire church in the city of Nicaea.

Constantine sent out the invitations, and the First Council of Nicaea was born.

Who was included in the council?

Emperor Constantine invited every Christian bishop to attend the council. Of the 1,800 bishops scattered across Rome, only a fraction of them made the trek to Nicaea, but we don’t know for sure how many came.

Eusebius of Caesarea, Athanasius of Alexandria, and Eustathius of Antioch all attended the council, and they each recorded a different number of bishops in attendance. Later church historians used Athanasius’ count of 318 (he gave the most precise number).

Not everyone who attended the council was a bishop. Constantine allowed each bishop to bring up to two priests and three deacons, so using Athanasius’ count, there could have been as many as 1,908 church leaders, plus Constantine and whoever accompanied him.

Key figures in the First Council of Nicaea

Obviously, there were hundreds of prominent leaders at the council, but some played much larger roles than others. Here are a few of the biggest players.

Alexander of Alexandria (also known as Saint Alexander I) led the opposition to Arianism. Prior to the council, Alexander had spent years trying to demonstrate that Arius’ beliefs were heretical and damaging to the church. He even officially excommunicated Arius, but other Christian leaders reinstated him. Alexander’s conflict with Arius is what ultimately led to the council’s formation.

Arius was a priest in Alexandria whose teachings about Christ largely led to the formation of the council. Arius argued his position that Christ was created by God and therefore not equal to God. The council deemed his teachings to be both heretical and incredibly harmful, so they exiled him to Illyria along with the only two council members who supported him. All of his writings were burned after the council, so we only know about his teachings from others.

Athanasius of Alexandria was a deacon and assistant to Alexander of Alexandria. After the council, he succeeded Alexander as archbishop of Alexandria, and spent most of his life trying to stamp out the remains of Arianism.

Hosius of Corduba (also known as Osius) was an influential bishop who supported Homoousion, the theological belief that Jesus is “one in being” and “of a single essence” with God. He supported Athanasius for years after the council, and was eventually excommunicated for it. (A future council ruled against the leaders of the Council of Nicaea.)

Eusebius of Caesarea, dubbed the Father of Church History, was present at the council and felt that the church was too hard on Arius. While he didn’t support Arius’ views himself, he was concerned about the divisiveness among the church’s leaders, and he was eventually excommunicated for being too sympathetic to Arius’ cause. He recorded details of the council in Life of Constantine.

Constantine the Great (also known as Flavius Valerius Aurelius Constantinus Augustus) was the first Roman emperor to convert to Christianity, and he called together the First Council of Nicaea. Constantine oversaw the proceedings but did not cast a vote.

Notably absent from the council was Pope Sylvester I. Unable to attend himself, the pope sent two representatives. Afterward, he supported the decision of the council.

What was decided at the First Council of Nicaea?

The First Council of Nicaea met for almost an entire month, from May 20–June 19. Their main objective was to resolve the conflict surrounding Arianism and get everyone on the same page about the Trinity. But while they had more than 300 of the most prominent Christian leaders in the room, they settled some other issues as well.

Over the course of that month, the council formed a statement of faith, likely working from one of several that had been circulating at the time, such as the Apostles Creed. This document was known as the Nicene Creed, and Emperor Constantine declared that anyone who did not sign the creed would be exiled. (Remember, all he really cared was that everyone agreed and got along.)

Arianism was deemed heretical

After Alexander and Arius each made their case to the church, the council presented the Nicene Creed, and with it, they sealed the fate of Arianism. The creed included lines specifically written to condemn Arianism and uphold Homoousion (the orthodox understanding of the Trinity).

Two bishops refused to sign the creed and sympathized with Arius. When Arius was exiled to Illyria, they got to tag along.

To put an end to Arianism once and for all, Emperor Constantine ordered that all of Arius’ works be burned, and his critics happily obliged. Constantine even ordered that if anyone was found with Arius’ writings, they would be put to death:

“In addition, if any writing composed by Arius should be found, it should be handed over to the flames, so that not only will the wickedness of his teaching be obliterated, but nothing will be left even to remind anyone of him. And I hereby make a public order, that if someone should be discovered to have hidden a writing composed by Arius, and not to have immediately brought it forward and destroyed it by fire, his penalty shall be death. As soon as he is discovered in this offence, he shall be submitted for capital punishment.”

(Despite their best efforts, Arianism still hung around for several hundred years.)

Easter got an official date

Since Jesus was crucified shortly after the Passover, early Christians relied on the Jewish lunar calendar to determine when to celebrate his resurrection.

After a couple hundred years though, some Christians began to doubt the Jewish calendar’s reliability. Contemporary Jews ignored the equinox, which threw off the lunar month. So some Christians wanted to create a new calendar that followed the Jewish calendar but accounted for the equinox. Not everyone agreed, though.

The First Council of Nicaea sided with those who wanted an independent Christian calendar, which caused some problems later on. (The new church calendar was a little too independent, and the church couldn’t agree on how to set or follow it. Plus Easter occasionally happened on the same day as the Passover.)

The council tried (and failed) to make peace with the Meletians

Remember Miletius, who may or may not have ordained Arius? He was excommunicated for disrupting the unity of the church because he refused to accept Christians who renounced their faith to avoid becoming martyrs, even after they repented. His “Church of the Martyrs” was a slap in the face to these Christians, and he amassed quite a following by the time of the council. He even ordained some of his own bishops.

The Council of Nicaea offered to recognize the Meletian bishops if they were “reordained” by other bishops.

Then they basically treated them as second-class bishops and continued to ignore Miletius.

So the Meletians sided with the Arians, and became a huge problem for decades.


The council also established a set of about 20 regulations and agreements about various church things, such as baptism, the eucharist (communion), and how to treat people who “lapsed” in their faith under persecution (these people were known as “Lapsi”). These “church laws” were known as canons.

Some scholars also debate whether or not the council determined the biblical canon as well, but there’s no solid evidence that they did. (Jerome appears to give a hint, but it’s pretty ambiguous.)

The Nicene Creed

At some point, most people have heard at least a line from the Nicene Creed. This ubiquitous statement of faith doesn’t quote Scripture, but it is based on the early church’s established understanding of Scripture. The creed was modified by the First Council of Constantinople, so it’s sometimes referred to as the Niceno-Constantinopolitan Creed.

Here’s the original creed, which the council signed together:

“We believe in one God,
the Father almighty,
maker of all things visible and invisible;
And in one Lord, Jesus Christ,
the Son of God,
begotten from the Father, only-begotten,
that is, from the substance of the Father,
God from God,
light from light,
true God from true God,
begotten not made,
of one substance with the Father,
through Whom all things came into being,
things in heaven and things on earth,
Who because of us men and because of our salvation came down,
and became incarnate
and became man,
and suffered,
and rose again on the third day,
and ascended to the heavens,
and will come to judge the living and dead,
And in the Holy Spirit.
But as for those who say, There was when He was not,
and, Before being born He was not,
and that He came into existence out of nothing,
or who assert that the Son of God is of a different hypostasis or substance,
or created,
or is subject to alteration or change
—these the Catholic and apostolic Church anathematizes.”

You may have already noticed the lines that specifically address Arianism, but that last line—”these the Catholic and apostolic Church anathematizes”—essentially promises to excommunicate anyone who holds to Arianism.

What happened after the council?

Despite having the backing of hundreds of bishops and the added authority of Emperor Constantine, the First Council of Nicaea didn’t immediately solve the church’s problems with Arianism.

There were already leaders in the church (even at the council) who were sympathetic to Arius, and Arianism continued seeping into the church, so much so that Constantine began to tolerate them (again, ironically, out of a desire for unity). On his deathbed, Constantine was even baptized by an Arian bishop (Eusebius of Nicomedia).

For a time, the church seemed to be on a pendulum, swinging between Arianism and Homoousion. Emperors after Constantine, including his son, Constantinus II, supported Arianism. Some of those who excommunicated Arius were excommunicated themselves.

Arius was even invited back into the church, but he died suddenly (and suspiciously) on his journey to be received by Alexander of Constantinople.

The church held several major councils in the centuries following the First Council of Nicaea (including the Second Council of Nicaea in 787), and several of these councils had to make a point of reaffirming the Nicene Creed.

The lasting impact of the Council of Nicaea

For the first time in the church’s history, the Council of Nicaea established a unified doctrine on the Father, the Son, and the Holy Spirit. And the Nicene Creed is still in use around the world today.

At a crucial moment in a fragile church, the Council of Nicaea may very well have prevented Christianity from self-destructing. While the rift remained for years to come, this formal act of unity helped set the healing process in motion.

Unfortunately, the Council of Nicaea also set a dangerous precedent for using the emperor’s authority to enforce church decisions. Many of the church leaders who Constantine supported would later see emperors turn against them, and for centuries, Christians would experience the consequences of uniting the state and the church.