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1. First Council of Nicaea
Defined the original Nicene Creed. The council defined the equality of God the Father and Our Lord Jesus His only begotten Son. It taught that Jesus was of the same substance as God the Father and not just merely similar. This is not clear in the bible but implied. The Council gives the definitive interpretation.
2. First Council of Constantinople
Explained and clarified the Nicene Creed, which is still used in the Catholic Church in the original form.
Defined the divinity of the Holy Spirit, which is derived but not defined in the Bible.
3. Council of Ephesus
Proclaimed the Virgin Mary as the Theotokos (Greek Η Θεοτόκος, “Mother of God”). As Our Lord followed the commandment to honor thy Father, He honors His mother. Motherhood here is seen in a traditional sense as co-creator, nurturer and carrier/bearer. Our Lady is can be thought of as a living chalice or tabernacle. This is the source of confusion and scorn. When a Catholic uses the term “Mother of God” we do not mean to suggest She created God, who is outside of time, and Mary is a creature created in time.
4. Council of Chalcedon
Explained and defined the two natures (divine and human) of Jesus Christ. Our Lord is fully God and fully human.
5. Second Council of Constantinople
Addressed the heresy of monophysitism
6. Third Council of Constantinople
Addressed the heresy of Monothelitism, reaffirming that Our Lord Jesus, being both fully God and fully human, had both human and divine wills.
7. Second Council of Nicaea
Declared against the iconoclasts that pictures of Our Lord and the Saints were to be allowed and used to stimulate the lived faith.
8. Fourth Council of Constantinople
This Council addressed the differences between Eastern and Western Churches in 869AD. This is the point where Greek Orthodox and Roman Catholics diverge. This council caused friction over two main issues: The Council refused the selection of a layman Photios to be Patriarch of Constantinople, who in turn had attacked the pope as a heretic, because he kept the word filioque in the creed (which referred to the Holy Spirit emanating from God the Father and the Son).
9. First Council of the Lateran
Confirmed bishops were not to be selected by secular leaders and the Emperor reserves the right to give secular appointments or honors.
10. Second Council of the Lateran
After the death of Pope Honorius II (1124–1130), two popes were elected by two groups of Cardinals. Sixteen cardinals elected Pope Innocent II, while others elected Antipope Anacletus II who was called the Pope of the Ghetto, in light of his Jewish origins. Then Council deposed the antipope and his followers. In important decisions regarding the celibacy of Catholic priests, clerical marriages of priests and monks, which up to 1139 were considered illegal, were defined and declared as non-existing and invalid. The Council met under Pope Innocent II in April 1139 and issued 30 canons.
11. Third Council of the Lateran
Established the two-third majority necessary for the election of a pope and outlawed simony, and the elevation to Episcopal offices for anyone under thirty. The council also ruled it illegal to sell arms or goods which could assist armaments to Muslim powers.
12. Fourth Council of the Lateran
Proclaimed the “Easter Duty”. Every Christian was required to go at least once a year on Easter to confession and to receive the Holy Eucharist. It also clarified the teaching of transubstantiation.
13. First Council of Lyon
The Council addressed the disposition of the heretical Frederick II, as German king and emperor, and decided the institution of the Octave of the Nativity of the Blessed Virgin.
14. Second Council of Lyon
Pronounced the conduct of papal conclaves and the privileges of a number of religious orders.
15. Council of Vienne
Pope Clement V solemnly opened the council with a liturgy, which has been repeated since in all Catholic ecumenical councils. He entered the Cathedral in liturgical vestments with a small procession and took his place on the papal throne. Patriarchs, followed by Cardinals, archbishops and bishops were the next in rank. The Pope gave a blessing to the choir, which intoned the Veni Sancte Spiritus. The council itself dealt with the suppression of the Knights Templar and discussed another Crusade to free the Holy Land.
16. Council of Constance
Settled a schism in the Church when concurrently there were three popes, each claiming legitimacy. The council ended the Western Schism, by deposing or accepting the resignation of the papal claimants and electing Pope Martin V. The council also condemned Jan Hus as a heretic.
17. Council of Basle/Ferra/Florence
Dealt with unity with the Eastern Churches and on the relationship between popes and ecumenical councils.
18. Fifth Council of the Lateran
Confirmed the truth that the soul of a human being lives forever. In a message equally applicable today the opening sermon announced that people must be transformed by holiness not holiness by the people.
19. Council of Trent
The council issued condemnations on what it defined as Protestant heresies and defined Church teachings in the areas of Scripture and Tradition, Original Sin, Justification, Sacraments, the Eucharist in Holy Mass and the veneration of saints. It issued numerous reform decrees. The Council entrusted to the Pope the implementation of its work, as a result of which Pope Pius V issued in 1566 the Roman Catechism, in 1568 a revised Roman Breviary, and in 1570 a revised Roman Missal, thus standardizing the varients of Holy Sacrifice of the Mass, hence why the Mass of Pius V is sometimes referred to as the Tridentine Mass.
20. First Vatican Council
The Council was convened in 1869 but was interrupted in 1870 because of military conflict. It issued definitions of the Catholic faith, the papacy and the infallibility of the Pope.