The Seven Ecumenical Councils
The continual, effective operation of Grace and the inspiration of the Holy Spirit in the Church is clearly shown in the holy councils of the Church. In these Councils, the Holy Spirit, as the "Conscience of the Church" guided and directed the holy fathers gathered in the truth and faith of Jesus Christ, as the Lord Himself had promised. The consent of the people themselves, both clergy and laity alike formed another link as to what is the "Conscience of the Church" because of the interaction of the Holy Spirit. The "Conscience of the Church" is a transcendent insight of divine force; it evaluates the decisions of the Synods (General Councils) and either places the seal to the formulation of disputed Truths or rejects all of its proceedings. The Church of Christ holds the vision of the entire Church - visible and invisible - which could foresee the errors of the assembly of bishops. Usually the official rejection of a synod is pronounced by another Synod that follows it, although this rejection is already consummated in the hearts and minds of believers universally.
In the first centuries, those who followed Our Lord Jesus Christ lived like brethren, comprising one Body: the Church, having one Head: Jesus Christ. Thus, when quarrels or misunderstandings arose, they resolved them by common agreement in a council of bishops and presbyters, upon the inspiration of the Holy Spirit (Acts 15:28-29). These meetings are called "Councils" (Acts 15:6-8).
During the time when the Holy Church was heavily persecuted in the Roman Empire, the councils were only local or regional. From the time of Constantine (Emperor), when the Church was freed from persecutions, however, general councils of the whole Church began to be convened, at which the bishops of the whole Christian world participated. Those general councils which were ratified as being truly Orthodox are known as "Ecumenical Councils". The word "Ecumenical" today has been appropriated by Protestants in such ways that the public in general normally think of "Ecumenical" as of Protestant origin when, in fact, it is not at all, but a misguided promotional angle to suit their own needs for what is actually a false church growth scheme. They are neither Canonical, nor even truly Christian; although there are many within them that are, in their hearts, Christian, following misguided contrived falsehoods.
Following the recognition of Christianity by Emperor Constantine, differences of opinion developed regarding the exact and correct doctrines which Christ had given to the church. Discord over these differences developed. Ecumenical Councils preached nothing new. They preached that which our Lord and Savior, Jesus Christ and His Apostles taught. They did not compose dogmas, but only gave explanation to those expressed in the Holy Scripture of Jesus Christ's Gospel message. Thus, for example, the Symbol of Faith is drawn entirely from Scripture.
The main reason for convening Ecumenical Councils was to define and correct heresies or disagreements in the faith.
A firm believer in systematic standardization, Emperor Constantine called a meeting of the Bishops and Clergy for the purpose of settling the differences in beliefs and doctrines. Known as the First Ecumenical Council, this meeting was held in Nicea in 325 A.D. sometimes referred to as 1st Nicea. It was attended by three hundred and eighteen men of the clergy including every Eastern Bishop of importance and four Western Bishops. Subsequently, six other Ecumenical councils were convened.
The First Ecumenical Council
At this First Ecumenical Council, the preeminence of Bishops of the three main centers of the Roman Empire (Rome, Alexandria and Antioch) was approved. As a mark of honor, the Bishop of Jerusalem was added. This council also condemned the heresy of Arius who denied the Divinity of Christ.
This first council proclaimed the true teaching concerning God the Father and God the Son, Jesus Christ. The council formulated canons regulating the church and drew up the first seven articles of the Creed.
The Second Ecumenical Council
At the Second Ecumenical Council, held at Constantinople in 381 A.D. (sometimes referred to as 1st Constantinople), the last five articles of the Creed were composed. The completed creed of twelve articles, which is the symbol of our faith and which is still being used, is called the Nicene Creed. The second council also rejected the teachings of Macedonius, condemning these teachings as a heresy against the Holy Spirit. Macedonius falsely taught that the Holy Spirit was created by God similarly to angels, being a spirit of a higher degree or order than the angels. At this council the words, "and in the Holy Spirit, the Lord, the Giver of life," were included into the Symbol of Faith. The 150 Bishops who attended also granted preeminence to the Bishop of Constantinople. The reason for the preeminence granted to Rome, Alexandria, Antioch, Jerusalem and Constantinople in the first two councils was due to the fact that the cathedrals of the Bishops in those cities were established by the Apostles themselves. Rome was looked upon as the former capital of the world and Constantinople as the new seat of the empire.
The Third Ecumenical Council
In 431 A.D. the Third Ecumenical Council was held in Ephesus. This council condemned the heresy of Nestorius who taught that our Lord was only a man with the divinity abiding in Him like in a temple. Nestorius called the Virgin Mary the Mother of Christ and not the Mother of God. Nestorius refused to acknowledge Mary as "Theotokos" (bearer of God), but would call her only "Christotokos". Two hundred Holy Fathers attended this council.
The Fourth Ecumenical Council
The Fourth Ecumenical Council was held 451 A.D. in Chalcedon with 630 Holy Fathers attending. This council's great work was its definition of Jesus Christ as the Second Person in the Trinity as True God and True Man with His divine and human natures distinct without confusion and inseparably united in One Person. This council condemned the Eutychius party who taught that Jesus was God only and that His divine nature absorbed the human one. This council also decreed that the Patriarch of Constantinople was the single head of the church in Eastern Europe.
The Fifth Ecumenical Council
In 553 A.D. the Fifth Ecumenical Council (sometimes referred to as 2nd Constantinople) convened at Constantinople with 160 church leaders attending. At this meeting the followers of Nestorius were excommunicated and their writings condemned. Nestorius taught that the Virgin Mary's title as Mother of God was erroneous. At this time, not only Nestorius, but Eutychius, Arius, Origen and others were reaffirmed as condemned for their errant teachings.
The Sixth Ecumenical Council
The Sixth Ecumenical Council, with 170 Holy Fathers attending, also met in Constantinople, in the year 680 A.D. (sometimes called the Council of Trullo). This council condemned the teachings of the Monotheists who acknowledged only the divine will of Christ while denying the human one. Also, this council dealt with moral questions, confirmed the canons of the various councils and the 85 Apostolic Canons, decreed that bishops shall not be married and that deacons and priests may not marry after their ordination. This council adjourned and reconvened in 691 A.D. at the Trula Palace. The council approved the canons of preceding councils.
The Seventh Ecumenical Council
The Seventh and last Ecumenical Council was convened by Empress Irene and was held at the site of the first council, Nicaea. This council took a stand against iconoclasm and defined the doctrine concerning images and their veneration (not worship) and ordered the images restored in churches; affirming and defined the veneration of holy ikons (icons). The personnel at this council was composed almost entirely of Byzantine representatives but Papal legates were also present. This council met in 787 (788 by some reckoning) A.D. with 367 attending. In the seven council meetings, about 2000 representatives of the undivided Christian church participated.
There was growing disagreement between Rome (Seat of the Roman Jurisdiction of the undivided Catholic Church) and the other Patriarchates. But the separation of the Roman Jurisdiction of the Catholic Church did not take place until 1054 A.D. with the final break due to a breach over Filioque dogma.
There were other local councils convened since those times and which are also canonical, such as the famous "Palamite Councils" of 1341 and 1351, the Council of Jerusalem of 1583 and the Councils of Constantinople of 1583 and 1593, are examples.
The authoritative body of the Orthodox Catholic Church from the East to the West remains the Ecumenical Synods of the One Undivided Church of the first thousand years, whose decisions have been preserved without change throughout its life, from the beginning of the Christian Church. The decisions of these Ecumenical Synods of the One Undivided Ecumenical Church have not been altered, changed or added to by the Orthodox Church.
[Excerpts of the 7 Ecumenical Councils from Celtic church in Germany, 2016. With thanks]