I've realized recently that it's very likely that a lot of readers of this blog - both LDS and Evangelical - aren't very familiar with church history. (By "church history" I mean here the history of Christianity in general, not just the small subset of it that's labeled "church history" in LDS circles.) For Latter-day Saints, most of this material isn't recognized as being a part of their heritage; instead, it's considered to be part of the Great Apostasy and so is seldom studied in any detail. Latter-day Saints commonly know only a very vague and rather slanted outline, one that can at times be misleading or outright erroneous. Evangelicals are in an even worse situation, because most of us have just as much or more ignorance on the topic, and considerably less excuse, since at least the first four ecumenical councils are a major part of our heritage. (There are seven ecumenical councils agreed upon by Roman Catholics, Orthodox, and Protestants, but Protestants generally deemphasize the last three and sometimes don't accept them as being truly ecumenical or as being equally valid with the first four.) So it occurred to me to do a brief review of some of the highlights as I see them. Well, I suppose 'brevity' is rather relative...
Anyway, the first ecumenical council was the First Council of Nicaea. Obviously, it was held in the city of Nicaea (now Iznik, Turkey). This council was called some years after Constantine became the sole emperor of the Roman world. Now, we can obviously disagree about whether Constantine was a genuine Christian at all, or whether he was good for Christianity, but the basic point is that Constantine converted around 312 and decreed a sort of religious toleration. Recognizing that the unity of the church would be good for the unity of the empire, he wanted Christians to get along and agree on their theology instead of fighting. (Whether or not Constantine's motives were the best, we can't forget that Jesus also wanted us to strive for unity.)
Now, at the time, there were a couple issues that were dividing Christians. The first was the date of Easter. For centuries already, some Christians had been celebrating Easter on a fixed date according to the Jewish calendar, while other Christians always celebrated Easter on a Sunday. The first group was prominent in Asia Minor (modern-day Turkey), where people who celebrated Easter that way said that they learned it from the apostle John. Because they celebrated it on the fourteenth day of the Jewish month of Nisan, they were called Quartodecimans, and the issue was called the Quartodeciman Controversy. When Constantine called bishops from all over the world together at Nicaea to resolve some of their issues, they eventually agreed to always celebrate Easter on a Sunday. To this day, both Latter-day Saints and Evangelicals follow the decisions of the Council of Nicaea on this, whether we realize it or not.
The other major issue that was debated at the Council of Nicaea was, ultimately, more important, and it can't be understood without some background. Long before this time, Christians had believed that God was incorporeal, impassible (not subject to suffering), immutable (unchangeable), and uncreated, whereas everything besides God was created out of nothing. That's the fundamental framework in which all of the debates were carried out. Christians by this time were also often comfortable in borrowing language from Greek philosophy as a way of describing God, a process that began in the New Testament period.
Around 313, a church elder named Arius in Alexandria, Egypt, began expressing some novel ideas. Arius saw that the church had long been referring to the Father as agennetos ('unbegotten') and the Son as gennetos ('begotten'). Arius decided that 'agennetos' was a description of the Father's ousia, or 'essence' - basically, the fundamental kind that the Father is. On the other hand, the Son was gennetos, and so Arius concluded that the Son and the Father had different essences. The Father was fully divine and eternal as God; but, Arius reasoned, then the Son would have to be a lesser level of divine. (Arius was influenced by Neoplatonic philosophy, which allowed for different levels of divinity.) In addition, since only God is eternal and uncreated, the Son would have to be created out of nothing - Arius described the Son as being "the perfect creation of God, but not like one among other creatures". As a result, one of Arius' slogans was, "There was [a time] when the Son was not". For Arius, the Son "is not eternal or co-eternal or equally self-sufficient with the Father".
Also, only God was unchanging, and so if Jesus was less divine and created, then Jesus would also be mutable (although several years after the outbreak of the controversy, Arius did describe Jesus as "immutable and unchanging"). Arius liked this - according to a study of early Arian views of salvation - because he viewed Jesus' life as primarily a moral example and thought that making Jesus capable of sin would make him easier for Christians to emulate. The downside, as opponents of Arianism have since pointed out, was that if Jesus was mutable, then we have no total assurance that he won't someday decide that he's tired of being our Savior and would prefer to leave us behind. Additionally, Arius' opponents hinted that Arianism was incompatible with their own belief in an unchanging God, since Arianism implied that God was once not a Father but later became a Father when he created his Son.
Finally, Arius believed in a form of divine incomprehensibility - that is, because God is infinite and every created mind is finite, therefore no created mind can fully understand God. As Arius' opponents pointed out, if Jesus is simply the highest created thing, then even he can't fully understand the Father - which raises serious questions about how he can perfectly reveal the Father to us as he claimed to.
Needless to say, Arius' views were controversial. His bishop, Alexander, was firmly opposed to Arius' ideas. Alexander instead held that "the Son exists eternally in dependence on the Father", and that the Son was eternally "begotten not out of nothing but out of the Father". In 318, Alexander called together a small local council of bishops to discuss the issue, and they excommunicated Arius. However, this didn't stop Arius. Arius was a very charismatic speaker who had a gift for poetry and song, and he spread his message by writing a collection of theological songs to familiar tunes; his collection was called the Thalia, and unfortunately we only have fragments of it today that were quoted by his opponents. Over the course of the next few years, Arius and his friends - especially people who, like Arius, had studied under a teacher named Lucian from the city of Antioch - gained enough support to put this controversy into the spotlight.
This Arian controversy was another of the major theological issues dealt with at the Council of Nicaea. (Note well that the issue of what books should be in the New Testament was not dealt with at Nicaea; most of it had been settled long already, and debate was beginning to settle down over a few pieces that eventually made it in.) Alexander attended the Council of Nicaea and brought along his protege, a short dark-skinned Egyptian deacon named Athanasius, who would become much more important later on. (Athanasius' opponents derogatorily mocked him as "the black dwarf".) At the Council of Nicaea, eventually all but two of the bishops agreed to condemn Arius and sign a statement of faith that the council had prepared: the Nicene Creed.
The Nicene Creed was structured in the same way that creeds had usually been structured since the very beginning: a statement about the Father, a (typically) longer statement about the Son, and then a statement about the Holy Ghost, occasionally with a few miscellaneous other things. (Our word "creed" comes from the Latin word 'credo', meaning "I believe", and simply means any statement of faith. For instance, the LDS Articles of Faith are a creed, regardless of whether Latter-day Saints like or dislike to think of them as such. In ancient Christianity, however, creeds were often instead called "symbols", referring to their use as ways of identifying Christians.) For example, consider to the second-century creed used at baptisms in Rome:
I believe in God the Father Almighty, and in Jesus Christ his only-begotten Son, our Lord, who was born from the Holy Spirit and the Virgin Mary; who was crucified under Pontius Pilate and was buried, and on the third day rose from the dead. He ascended into the heavens and is seated at the right hand of the Father, from where he will come to judge the living and the dead. And [I believe] in the Holy Spirit, the holy church, the forgiveness of sins, the resurrection of the flesh, and everlasting life.
And then the much briefer third-century (or early fourth-century creed) recovered from the Der Balizeh papyrus fragment:
I believe in God the Father Almighty and in his only-begotten Son our Lord Jesus Christ, and in the Holy Spirit, and in the resurrection of flesh in the holy catholic church.
[Note: the word "catholic" comes from the Greek kat' holou, meaning "according to the whole"; in other words, it talks about the whole universal church rather than smaller sectarian movements.]
The original Nicene Creed, in its statement about the Son, said that he was 'of the same essence as the Father', using the Greek word homoousios ('of the same ousia [essence]'), which was controversial because it had once been used by people - the Sabellians, Patripassionists, or modalists - who tried to say that the Father and the Son were the same person (which was not what the Council of Nicaea said, nor what Nicene Christians have believed ever since). The Nicene Creed then stated:
We believe in one God the Father Almighty, Maker of all that is, seen and unseen. And [we believe] in one Lord Jesus Christ, the Son of God, the Only-Begotten begotten from the Father - that is, from the essence of the Father - God from God, light from light, true God from true God, begotten not made, of the same essence as the Father, through whom all things came to be, both those in heaven and those in earth. For us and for our salvation, he came down and became incarnate, became human, suffered and rose up on the third day, ascended into the heavens, is coming to judge the living and the dead. And [we believe] in the Holy Spirit.
Note that it introduced a contrast between 'begotten' (gennethenta) and 'made' (poiethenta) in saying that the Son was "begotten, not made". Appended to the creed was a series of explicit condemnations of Arian catchphrases:
And those who say "there once was when he was not", and "before he was begotten he was not", and that he came to be from things that were not, or from another hypostasis or ousia, affirming that the Son of God is subject to change or alteration - these the catholic and apostolic church anathematizes.
Unfortunately, the Council of Nicaea did not end the debate. Three years later, Alexander died and Athanasius became bishop of Alexandria in his place. In the meantime, Arius' supporters were regrouping. There were some who thought Arius went too far and so they instead tried to stake out various middle positions between the Nicene position and the Arian position; others tried to ban the word 'ousia' from theological discussions entirely; and still others thought that Arius wasn't quite radical enough. Constantine eventually decided that Arius had gotten a raw deal and not only recalled him from exile but also ordered that his excommunication be overturned. Arius died suddenly the day before he would've been readmitted to the sacrament. It's worth noting that when Constantine eventually got baptized before he died, the baptism was performed by an Arian bishop named Eusebius of Nicomedia (not to be confused with the church historian Eusebius of Caesarea, who took a middle position).
After Constantine died, the emperors that came after him made life pretty hectic. Some favored the Nicene position and tried to suppress Arianism; some favored the Arian position and tried to suppress Nicene Christianity; and one ('Julian the Apostate') converted to paganism and tried to encourage Christian infighting. Before dying in 373, Athanasius had been exiled five times because of his refusal to compromise with views that, as he saw it (and I think rightly), would've made salvation impossible if they were true. Because of his uncompromising stance in times when the emperors made things very hard on people like him, the saying arose, Athanasius contra mundum - "Athanasius against the world". (Athanasius won.)
After Arius died, two of his radical followers - Aetius of Antioch and Eunomius of Cyzicus - carried on his legacy. They were notable for denying divine incomprehensibility and in fact boasting that they understood God as well as God understood himself. Eunomius said about Jesus that he was "not without an act of begetting prior to his own existence", "not uncreated", and "not without beginning", and that the Son "does not partake of the status of the one who begot him or share with any other the Father's essence or his kingdom"; he also referred to the Holy Ghost as being the Son's most glorious creation, as being "the first and most mighty work of the Only-Begotten". Aetius' surviving work, the Syntagmation, includes a series of brief philosophical arguments in sequence - for instance:
If the Deity is ingenerate in essence, what was generated was not generated by sundering of essence but he posited it by his power. For no reverent reasoning permits the same essence to be both generate and ingenerate.
On the other hand, in addition to Athanasius, they were resisted by a trio called the Cappadocian Fathers, a group of three bishops from the region of Cappadocia in Asia Minor: Basil of Caesarea, his brother Gregory of Nyssa, and their close friend Gregory of Nazianzus, who eventually become (quite against his will) the bishop of the imperial city Constantinople for a while. Basil wrote a lengthy response to Eunomius, the Adversus Eunomium, as well as an important treatise called On the Holy Spirit. Gregory of Nyssa delivered five famous sermons - his Theological Orations - and is also known for a theological letter addressed to his friend Ablabius. These Greek-speaking Christians pioneered further developments in theology by distinguishing between two Greek words, ousia and hypostasis, which just a few decades earlier had usually been used as synonyms. They explained that while the Father, the Son, and the Holy Ghost as three hypostases (roughly, 'persons' or 'individual substances'), they are all one and the same ousia ('essence'). This matched up quite nicely with the formula of 'una substantia, tres personae' ("one substance, three persons") that had been independently reached in the Latin-speaking West long before Nicaea.
Eventually, Gregory of Nazianzus chaired the second ecumenical council, the First Council of Constantinople, which met in 381 to re-condemn all forms of Arianism and also another view that had risen in the meantime. Its adherents were called Macedonians (after Macedonius, their leader), and their opponents called them Pneumatomachians (literally, "those who fight against the Spirit"), because these people focused on denying that the Holy Ghost was of the same ousia as the Father and the Son. This council also condemned Apollinarianism, the beliefs of Apollinaris of Laodicea. In Greek philosophy, the soul was often divided into several parts, the highest one being the nous ('mind' or 'rational soul'). Apollinaris maintained that while Jesus had a human body and a human soul, he lacked a human nous. It was a commonly held dictum, however, that Christ's atonement could not affect any part of us that he did not share; in other words, "that which is not assumed, is not healed". Hence, only if Christ was completely and fully human - including having a human nous - could he redeem our whole humanity from sin.
The Council of Constantinople issued a revision of the Nicene Creed, which is essentially the one still in use today. The major difference was a major expansion of the statement about the Holy Ghost. Where the Council of Nicaea had only said, "And [we believe] in the Holy Ghost", the Council of Constantinople said:
And [we believe] in the Holy Ghost, the Lord, the Life-giver, who proceeds from the Father, who with the Father and Son is worshipped and glorified, who spoke through the prophets.
Within the Roman Empire, Arianism soon died out; it survived longer in western and northern Europe because the barbarian tribes were evangelized by a half-Goth Arian missionary named Ulphilas, but eventually those tribes were convinced to abandon Arianism. Having formally established the consubstantiality (that is, the 'same-essence-ness') of the three persons of the Godhead - the Trinity - controversies eventually arose elsewhere.
Many Christians had taken to giving Mary certain titles, one of the most prominent ones being 'Theotokos' (literally, "God-bearer" or "birthgiver-to-God", but sometimes translated a bit loosely as "mother of God"). The focus wasn't so much to honor Mary, but to use this title as a way of redirecting focus to Jesus as divine. This practice goes back even before the Council of Nicaea; at a smaller council held in Antioch earlier in the year 325, the bishops declared that "God the Word" was "born and made flesh out of Mary the Theotokos". In the early fifth century, however, it became a point of controversy. There was a bishop of Constantinople by the name of Nestorius, and he objected to this word. Nestorius was a very firm believer in divine impassibility, God's freedom from being affected by suffering. So Nestorius wanted to draw a distinction between the divine Word and the human Jesus; Jesus suffered on the cross, but the Word didn't. Nestorius was very insistent that we maintain a distinction there. The Word and Jesus were very closely associated, but they had to remain distinct. As a result, Nestorius had to object to talking about Mary as theotokos. She was the mother of the human Jesus but not the mother of the Word - and so she did not, according to Nestorius, actually give birth to a divine person. He was willing to honor her as Christotokos ('Christ-bearer'), but not as Theotokos. Nestorius proclaimed:
If anyone says that Emmanuel is "truly God" and not instead "God with us" [...] and if anyone calls Mary "the mother of God the Word" and not instead "mother of him who is Emmanuel" [...] let him be anathema!
Nestorius was opposed by the Alexandrian bishop at the time, a man named Cyril, who held instead that Jesus is the Word. Cyril argued that although Jesus is both divine and human, still it's one and the same subject who is both divine and human; there's only one, not two. (Otherwise, the one who was sent from heaven is not the one who suffered for us; only a very special human suffered for us, which isn't enough to save us or to exalt us. So for Cyril and others, this was no arcane dispute; it made the difference between a Jesus who can save and a Jesus who can't.) He also supported calling Mary the Theotokos as a way of reminding ourselves of that.
The issue was settled at the third ecumenical council, the Council of Ephesus, which met in 431 and endorsed Cyril's view, rejecting Nestorius and removing him as bishop. It upheld the appropriateness of calling Mary 'theotokos' and maintained that the very same Jesus was both fully divine and fully human. This council's rulings include a reference to "the disgusting, perverted views of Nestorius". (This was all resisted by many Syriac-speaking Christians, who rejected the council and broke communion with the rest of the church; today, their position is embodied in the Assyrian Church of the East.) After the council, Cyril still had some concerns about the orthodoxy of the Antiochene bishop John, and so they reached an agreement in 433 called the Formula of Union, which declared that:
We confess, then, our Lord Jesus Christ, the only-begotten Son of God, perfect God and perfect man of a rational soul and body, begotten before all ages from the Father in his divinity, the same in the last days for us and for our salvation born of Mary the virgin according to his humanity - one and the same being of the same essence of the Father in divinity and of the same essence with us in humanity, because a union of two natures took place. Therefore we confess one Christ, one Son, one Lord. According to this understanding of the unconfused union, we confess the Holy Virgin to be Theotokos because God the Word took flesh and became human and from his very conception united to himself the temple he took from her.
What this council didn't do was go very far into the details of exactly how Jesus was both divine and human. A distinguished monk and church elder in Constantinople by the name of Eutyches hated Nestorianism and held a very extreme opposing view - or, at the very least, phrased his views in that way. Eutyches talked of Jesus' humanity being swallowed up in his divinity like a drop of water in the ocean. In 448, a small council headed by Flavian, the bishop of Constantinople, and supported by an Antiochene bishop named Domnus examined Eutyches and, finding his views too extreme, excommunicated him. The Roman bishop Leo agreed and sent Flavian a famous letter called the Tome, in which Leo argued that Jesus had two natures, a divine nature and a human nature, in one person:
To pay off the debt of our state, invulnerable nature was united to a nature that could suffer, so that in a way that corresponded to the remedies we needed, one and the same "mediator between God and humanity, the man Christ Jesus" could both on the one hand die and on the other be incapable of death. Thus was true God born in the undiminished and perfect nature of a true man, complete in what is his and complete in what is ours. By "ours" we mean what the Creator established in us from the beginning and what he took upon himself to restore. There was in the Savior no trace of the things which the deceiver brought upon us, and to which deceived humanity gave admittance. His subjection to human weaknesses in common with us did not mean that he shared our sins. He took on the form of a servant without the defilement of sin, thereby enhancing the human and not diminishing the divine. For that self-emptying whereby the Invisible rendered himself visible, and the Creator and Lord of all things chose to join the rank of mortals, spelled no failure of power: it was an act of merciful favor. So the one who retained the form of God when he made humanity, was made man in the form of a servant. Each nature kept its proper character without loss; and just as the form of God does not take away the form of a servant, so the form of a servant does not detract from the form of God. [...] As God is not changed by showing mercy, neither is humanity devoured by the dignity received.
However, Cyril's successor in Alexandria - a fellow named Dioscorus - was much less pleased with the results of Flavian's council. In 449, he chaired another council of Ephesus which reinstated Eutyches and deposed both Flavian and Domnus; this council supported Eutyches' view that in Christ there is only one nature, for which reason it is called monophysitism. The council also refused to read Leo's letter, in response to which Leo irately labeled it a Latrocinium - a "robber synod", by which name it's still known to this day. During this council, Flavian was physically assaulted by Eutyches and Dioscorus; Flavian died from his injuries three days later.
Although Dioscorus' council in Ephesus had been called together by the emperor Theodosius II, its decisions were so controversial that it wasn't accepted as an ecumenical council. After Theodosius died in 450 and his sister Pulcheria took the reigns of power and married a general named Marcian, who then became the new emperor, Marcian called a new council together in 451 at Chalcedon, a settlement across the Bosphorus Strait from Constantinople. (Today, both Constantinople and Chalcedon are part of Istanbul, Turkey.) This fourth ecumenical council, the Council of Chalcedon, put Dioscorus on trial and excommunicated him while vindicating Flavian and Domnus; it also reiterated the condemnation of Eutyches and accepted Leo's Tome. The Council of Chalcedon also drafted a powerful statement of faith:
So, following the saintly fathers, we all with one voice teach the confession of one and the same Son, our Lord Jesus Christ; the same perfect in divinity and perfect in humanity, the same truly God and truly man, of a rational soul and body; of the same essence with the Father as regards his divinity and of the same essence with us as regards his humanity; like us in all respects except for sin; begotten before the ages from the Father as regards his divinity, and in the last days the same for us and for our salvation from Mary the virgin Theotokos as regards his humanity; one and the same Christ, Son, Lord, Only-Begotten, acknowledged in two natures which undergo no confusion, no change, no division, no separation. At no point was the difference between the natures taken away through the union, but rather the property of both natures is preserved and comes together in a single person and a single hypostasis; he is not parted or divided into two persons, but is one and the same only-begotten Son, God, Word, Lord Jesus Christ, just as the prophets taught from the beginning about him, and as the Lord Jesus Christ himself instructed us, and as the creed of the fathers handed it down to us.
However, just as with the previous ecumenical council, the fourth was rejected by many Christians, especially Coptic Christians who felt that it denied their Alexandrian heritage. As a result, they too broke communion. The rest of the church, however, has remained Chalcedonian to this day; Roman Catholicism, Eastern Orthodoxy, and Protestantism are all traditions within the umbrella of Chalcedonian Christianity. All of them are agreed in believing in one God in three persons - Father, Son, and Holy Ghost - the second of whom became human without ceasing to be God in the incarnation, and who now unites in his one person two natures without altering either.
An even briefer survey of later developments: The fifth ecumenical council, which became the Second Council of Constantinople, was called by the Emperor Justinian in 553 and rehashed the issues of Chalcedon; in particular, it condemned the writings of three deceased theologians: Theodore of Mopsuestia, Theodoret of Cyrrhus (or 'of Cyrus'), and Ibas of Edessa, all of whom were believed to lean too strongly in the direction of Nestorius. This failed to reunite the monophysites with the Chalcedonians as Justinian likely had hoped.
After the rise of Islam (Muhammad fled to the Arabian city of Yathrib in 622, renaming it Medina, and then within the next decade captured the Arabian holy city of Mecca, dying there in 632), Justinian's quest was continued by the Roman emperor Heraclius, who endorsed a new compromise position called monothelitism, from the Greek words meaning "one will". Monothelites accepted the Chalcedonian view of two natures in one person, but insisted that the one person had only one will and one activity/energy. This was viewed by others, however, as irreconcilable with Jesus' prayer, "Not my will but yours be done." During a period when the monothelite view was dominant in Constantinople, Maximus the Confessor - a noted dyothelite (as he believed in two wills in Christ, one divine and one human) - was put on trial in both 658 and 662; in 662, he was tortured, had his tongue cut out, had his right hand sliced off, and was then sent into exile, where he died shortly thereafter. In 680-681, the sixth ecumenical council met in Constantinople. This Third Council of Constantinople vindicated Maximus and endorsed dyothelitism, ending the monothelite controversy. This council "proclaim[ed] equally two natural volitions or wills in him [Christ] and two natural principles of action which undergo no division, no change, no partition, no confusion, in accordance with the teaching of the holy fathers".
The next major controversy that erupted concerned icons, or images of Christ or various other Christian figures. In 726, the emperor Leo III the Isaurian ordered that an icon of Christ over a palace gate be taken down and replaced with simply a cross, perhaps because of a fear that his military losses to Muslim forces were God's punishment for excessive veneration of images. In 730, Leo made a decree forbidding the veneration of icons. His policy was continued by his son and successor Constantine V, who called a council of bishops in Hieria that endorsed this policy of iconoclasm (lit., "icon-breaking") but lacked representation from any of the five most prominent bishops (those of Rome, Constantinople, Alexandria, Antioch, and Jerusalem). This position was strongly rejected by a Syrian Chalcedonian church elder named John of Damascus, who wrote numerous treatises defending icons. After Constantine's son Leo died and Leo's wife Irene took the reins of power, space was opened up for a further consideration of the issue. Irene summoned the bishops to a seventh ecumenical council back at Nicaea in the year 787. At this Second Council of Nicaea, the bishops rejected the iconoclast position and supported instead the iconodules, those who venerated icons.
Those are accepted by Roman Catholics, Orthodox, and Protestants as the seven ecumenical councils. Roman Catholics have others, running through Vatican II, but none of those are accepted by the Orthodox Church - and, as mentioned before, aren't recognized by Evangelicals either.