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 325
Council of Nicaea (1st Ecumenical Council) presided over by Emperor Constantine. The Nicene Creed was developed.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.3. Council of Ephesus 431
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.
