There have been 21 councils of the Catholic Church (17 before 1453). A council is an assembly of theologians and church dignitaries called together to discuss the doctrine of the church. When Christianity took hold in the Roman Empire, doctrine wasn't fixed. Problems of interpretation initially presented themselves when trying to explain the divine and humans aspects of Christ without resorting to pagan concepts. Later councils dealt with church hierarchy and behavior.
A council that has not been convened legally is called a conciliabula, conventicula or latrocinia, according to the Catholic Encyclopedia.
1. Council of Nicaea 325Council of Nicaea
(1st Ecumenical Council) presided over by Emperor Constantine
. The Nicene Creed
The conflict leading to this council was Arianism, which was condemned. The religious concept of homoousian triumphed.
2. Council of Constantinople I 381
First Council of Constantinople
accepted the homoean concept, condemned Arianism. Convened by Theodosius I
Council of Ephesus (3rd Ecumenical Council) presided over by St. Cyril of Alexandria representing Pope Celestine I, renewed the condemnation of Pelagius
. Convened by Theodosius II
Nestorianism condemned. Mary was declared Theotokos (mother of God).
At the second council of Ephesus (not counted as an ecumenical council) Monophysitism was adopted.
4. Council of Chalcedon 451Council of Chalcedon
(4th Ecumenical Council) -- 150 bishops under Pope Leo the Great and the Emperor Marcian defined the two natures of Christ (against Eutyches). Monophysitism was rejected.
5. Second Council of Constantinople 553
Second Council of Constantinople (5th Ecumenical Council), under Pope Vigilius and Emperor Justinian I
, condemned the errors of Origen and Nestorianism.
6. Third Council of Constantinople 680-681Third Council of Constantinople (6th Ecumenical Council), under Pope Agatho and the Emperor Constantine Pogonatus. Condemned Monothelism.
7. Second Council of Nicaea 787Second Council of Nicaea (7th Ecumenical Council), convoked by Emperor Constantine VI and his mother Irene, under Pope Adrian I. Condemned iconoclasts.
8. Fourth Council of Constantinople 869Fourth Council of Constantinople (8th Ecumenical Council), under Pope Adrian II.
9. First Lateran Council 1123First Lateran Council (9th Ecumenical Council) held at Rome under Pope Callistus II.
10. Second Lateran Council 1139Second Lateran Council (10th Ecumenical Council), held at Rome under Pope Innocent II.