A Church Council is an official ad hoc gathering of representatives to settle Church business. Such Councils are called rarely and are not the same as the regular gatherings of church leaders (synods, etc). An ecumenical council is one at which the whole Church is represented. The three major branches of the Church (Orthodox, Catholic, Protestant) recognize seven ecumenical councils: Nicea (325), Constantinople (381), Ephesus (431), Chalcedon (451), Constantinople II (553), Constantinople III (680), Nicea II (787). Further ecumenical councils were rendered impossible by the widening split between Eastern (Orthodox, Greek-speaking) and Western (Catholic, Latin-speaking) Churches, a split that was rendered official in 1054 and has not yet been healed.
In addition to these universally-acknowledged councils, the Catholic Church recognizes a further fourteen ecumenical councils: Constantinople IV (869-70), Lateran I (1123), Lateran II (1139), Lateran III (1179), Lateran IV (1215), Lyons I (1245), Lyons II (1274), Vienne (1311-12), Constance (1414-18), Florence (1438-45), Lateran V (1512-17), Trent (1545-63), Vatican I (1869-70), Vatican II (1965). But these were councils of only the Catholic Church, and are not recognized by the Orthodox or Protestant Churches.
In 324 Constantine became sole ruler of the Roman Empire, reuniting an empire that had been split among rival rulers since the retirement of Domitian in 305. Constantine, the first Christian emperor, reunified the empire but found the Church bitterly divided over the nature of Jesus Christ. He wanted to reunify the Church as he had reunified the Empire. The major dispute was over the teaching of Arius, but there were other doctrinal issues also.
Constantine summoned the bishops at imperial expense to Nicea, 30 miles from his imperial capital in Nicomedia. Here they were to settle their differences in a council over which he presided. The council rejected Arianism. The Council issued a creed based upon an existing baptismal creed from Syria and Palestine. This Nicene creed reads:
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 of 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 were made.
Who for us men and for our salvation came down and became incarnate, and was made man, suffered and rose on the third day, And ascended into heaven, And is coming with glory to judge living and dead, And in the Holy Spirit.
But those who say, There was when the Son of God was not, and before he was begotten he was not, and that he came into being from things that are not, or that he is of a different hypostastis or substance, or that he is mutable or alterable--the Catholic and Apostolic Church anathematizes.
The Council also issued a set of canons, primarily dealing with church order.
The second council met in Constantinople, the new imperial capital. The council issued a new creed, probably based upon another baptismal creed from Jerusalem or Antioch, which in turn was an expression of the faith expressed in the Nicene Creed adopted in 325. This Constantinopolitan Creed reads:
We believe in one God the Father Almighty, Maker of heaven and earth, and of all things visible and invisible.
And in one Lord Jesus Christ, the only-begotten Son of God, begotten of the Father before all worlds, God of God, Light of Light, Very God of Very God, begotten, not made, being of one substance with the Father by whom all things were made; who for us men, and for our salvation, came down from heaven, and was incarnate by the Holy Spirit of the Virgin Mary, and was made man, and was crucified also for us under Pontius Pilate. He suffered and was buried, and the third day he rose again according to the Scriptures, and ascended into heaven, and sitteth on the right hand of the Father. And he shall come again with glory to judge both the quick and the dead, whose kingdom shall have no end.
And we believe in the Holy Spirit, the Lord and Giver of Life, who proceedeth from the Father [and the Son*], who with the Father and the Son together is worshipped and glorified, who spoke by the prophets. And we believe one holy catholic and apostolic Church. We acknowledge one baptism for the remission of sins. And we look for the resurrection of the dead, and the life of the world to come. Amen.
* and the Son (Lat. filioque) was added later. The Orthodox Church does not recognize this addition.
Later the Western Church unilaterally added a single word to the Creed, inserting Filioque “and the Son” to the statement about the Spirit, so as to read “the Spirit...proceeds from the Father and the Son.” In 867 the Patriarch of Constantinople declared Rome heretical for this clause. To this day the Western Church (Catholic and Protestant) accepts the filioque clause, while the Eastern Church (Orthodox) does not. With the exception of this clause, the Nicene Creed remains one of the eceumenical creeds, a creed recognized by all the Church. Any church that rejects the Nicene Creed is deemed heretical.
During the Middle Ages this creed became called the Nicene Creed, as it is known to this day.
Condemned Nestorius and his teaching (Nestorianism) that Christ had two separable natures, human and divine. Declared Mary to be theotokos (lit. God-bearer, i.e. Mother of God) in order to strengthen the claim that Christ was fully divine against those who called her merely Christotokos (Christ-bearer).
Issued the Chalcedonian Formula, affirming that Christ is two natures in one person.
Therefore, following the holy Fathers, we all with one accord teach men to acknowledge one and the same Son, our Lord Jesus Christ, at once complete in Godhead and complete in manhood, truly God and truly man, consisting also of a reasonable soul and body; of one substance (homoousios) with the Father as regards his Godhead, and at the same time of one substance with us as regards his manhood; like us in all respects, apart from sin; as regards his Godhead, begotten of the Father before the ages, but yet as regards his manhood begotten, for us men and for our salvation, of Mary the Virgin, the God-bearer (theotokos); one and the same Christ, Son, Lord, only-begotten, recognized in two natures, without confusion, without change, without division, without separation; the distinction of natures being in no way annulled by the union, but rather the characteristics of each nature being preserved and coming together to form one person and subsistence (hypostasis), not as parted or separated into two persons, but one and the same Son and Only-begotten God the Word, Lord Jesus Christ; even as the prophets from earliest times spoke of him, and our Lord Jesus Christ himself taught us, and the creed of the Fathers has handed down to us.
Condemned the Three Chapters, a compendium of the writings of Theodore of Mopsuestia, Theodoret of Cyrus, and Ibas of Edessa. These three were advocates of Antiochene theology, emphasizing Christ’s humanity at the expense of his deity. Their opponents held Alexandrian theology emphasizing Christ’s deity.
Condemned monothelitism (Christ has a single will), affirming that Christ had a human will and a divine will that functioned in perfect harmony.
Declared that icons are acceptable aids to worship, rejecting the iconoclasts (icon-smashers)
Leo Donald Davis, The First Seven Ecumenical Councils (325-787): Their History and Theology. Collegeville, Minn.: Liturgical Press, 1983.
— Bernard Bell