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First Council of Nicaea (325) 
- Called by Emperor Constantine to resolve the Arian controversy, which challenged the orthodox doctrine of the Trinity. 
The council affirmed the divinity of Jesus Christ and produced the Nicene Creed.

First Council of Constantinople (381) 
- Called to deal with the issue of Arianism and the nature of the Holy Spirit. 
The council affirmed the divinity of the Holy Spirit and revised the Nicene Creed to include a fuller expression of the Trinity.

Council of Ephesus (431) 
- Called to address the teachings of Nestorius, who denied the unity of Christ's divine and human natures. 
The council affirmed the doctrine of Mary as "Mother of God" and condemned Nestorianism.

Council of Chalcedon (451) 
- Called to address the issue of the nature of Christ and the teachings of Eutyches, who believed that Christ had only one nature. 
The council affirmed the doctrine of two natures in Christ, fully divine and fully human.

Second Council of Constantinople (553) 
- Called to address the teachings of Nestorianism and the Three Chapters controversy, which had caused division in the Eastern Church. 
The council affirmed the orthodox doctrine of the Trinity and condemned Nestorianism and the Three Chapters.

Third Council of Constantinople (680-681) 
- Called to address the teachings of Monothelitism, which denied that Christ had two wills (divine and human). 
The council affirmed the doctrine of two wills in Christ and condemned Monothelitism.

Second Council of Nicaea (787) 
- Called to address the issue of iconoclasm, which had caused division in the Eastern Church. 
The council affirmed the use of icons in worship and condemned iconoclasm.

Fourth Council of Constantinople (869-870) 
- Called to address the issue of Photianism, which challenged the authority of the pope and the Western Church. 
The council affirmed the authority of the pope and condemned Photianism.

First Lateran Council (1123) 
- Called to address issues related to the investiture controversy, which involved the power struggle between 
the pope and the Holy Roman Emperor. The council affirmed the pope's authority in the appointment of bishops and other church officials.

Second Lateran Council (1139) 
- Called to address issues related to clerical celibacy and the Holy Roman Emperor's interference in Church affairs. 
The council affirmed the practice of clerical celibacy and condemned the Holy Roman Emperor's interference.

Third Lateran Council (1179) 
- Called to address issues related to the election of popes and the regulation of Church affairs. 
The council established procedures for the election of popes and affirmed the authority of the pope in Church affairs.

Fourth Lateran Council (1215) 
- Called to address issues related to the nature of Christ and the authority of the Church. 
The council affirmed the doctrine of transubstantiation and established the sacrament of confession.

First Council of Lyon (1245) 
- Called to address issues related to the power struggle between the papacy and the Holy Roman Empire. 
The council affirmed the pope's authority over the Holy Roman Empire and established the office of papal legate.

Second Council of Lyon (1274) 
- Called to address issues related to the power struggle between the papacy and the Holy Roman Empire, as well as the issue of the Filioque clause in the Nicene Creed. 
The council affirmed the pope's authority over the Holy Roman Empire and established a temporary union between the Eastern and Western Churches, but this union ultimately proved unsuccessful.

Council of Vienne (1311-1312) 
- Called to address issues related to the Knights Templar and the role of the papacy in Church affairs. 
The council condemned the Knights Templar and affirmed the pope's authority over Church affairs.

Council of Constance (1414-1418) 
- Called to address issues related to the Great Schism, which had caused division within the Church. 
The council deposed three rival popes and elected a new pope, thus ending the Great Schism.

Council of Basel/Ferrara/Florence (1431-1445) 
- Called to address issues related to the power struggle between the papacy and the Holy Roman Empire, 
as well as the issue of the Filioque clause in the Nicene Creed. The council established a temporary union 
between the Eastern and Western Churches, but this union ultimately proved unsuccessful.

Fifth Lateran Council (1512-1517) 
- Called to address issues related to Church reform and the threat of the Ottoman Empire. 
The council established reforms in Church governance and affirmed the need to defend against the Ottoman threat.

Council of Trent (1545-1563) 
- Called to address issues related to the Protestant Reformation, which had caused division within the Church. 
The council affirmed Catholic doctrine on the Eucharist, the sacraments, and justification, and established reforms in Church governance and discipline.

First Vatican Council (1869-1870) 
- Called to address issues related to the role of the papacy and the relationship between faith and reason. 
The council affirmed the doctrine of papal infallibility and the Church's authority in matters of faith and morals.

Second Vatican Council (1962-1965) 
- Called to address issues related to Church reform and renewal in the modern world. 
The council affirmed the importance of ecumenism, the liturgy, and the role of the laity in the Church, among other reforms.

assistant: The first ecumenical council was held in Nicaea in 325. 
    The second ecumenical council was held in Constantinople in 381.
    The third ecumenical council was held in Ephesus in 431. 
    The fourth ecumenical council was held in Chalcedon in 451. 
    The fifth ecumenical council was held in Constantinople in 553. 
    The sixth ecumenical council was held in Constantinople in 680.  
    The seventh ecumenical council was held in Nicaea in 787. 

There have been 7 ecumenical councils in total. The first four were called into being by the Roman Emperor Constantine I and were focused on theological issues such as Arianism, which is a Christian belief that Jesus Christ is subordinate to God the Father and not equal in power or eternity. The next three (the Fifth, Sixth, and Seventh) were called into being by the Byzantine Empire’s Justinian I and his successors.

prompter: What was decided at the first ecumenical council, if that is known?

assistant: Yes, the decisions made at the First Ecumenical Council are well-documented. The First Ecumenical Council was held in Nicaea in 325 AD, and it was convened by the Roman Emperor Constantine I to address the issue of Arianism, a heretical teaching that denied the divinity of Jesus Christ.

The Council was attended by over 300 bishops, and after much discussion and debate, it produced the Nicene Creed, which affirmed the divinity of Jesus Christ and his equality with God the Father. The Council also established a uniform date for the celebration of Easter, which was to be the first Sunday following the first full moon after the spring equinox.

In addition to these doctrinal and liturgical decisions, the Council also established a number of canons, or rules, governing the organization and operation of the Church. For example, the Council established guidelines for the ordination of bishops, set limits on the jurisdiction of various bishops, and established rules for the resolution of disputes between clergy members.