The First Ecumenical Council of Nicaea (AD 325) This Council, the first Ecumenical Council of the Catholic Church, was held in order to bring out the true teaching of the Church as opposed by the heresy of Arius. It formally presented the teaching of the Church declaring the divinity of God the Son to be one substance and one nature with that of God the Father. There were twenty canons drawn up, in which the time of celebrating Easter was clarified and a denunciation of the Meletian heresy made, also various matters of discipline or law were dealt with and several decisions advanced. From this Council we have the Nicene Creed.
The First Ecumenical Council of Constantinople (AD 381) Again the true faith was maintained against the Arians. Answer was also given against the Apollinarian and Macedonian heresies. In answering the latter which denied the Godhead of the Holy Spirit, the dogma of the Church was again stated and the words inserted into the Nicene Creed declaring the truth that the Holy Spirit proceeded from both the Father and the Son.
The Ecumenical Council of Ephesus (AD 431) The third General Council of the Church defined the universal dogma that the Blessed Virgin is the Mother of God and presented the teaching of the truth of one divine person in Christ. The Council was convened against the heresy of Nestorius.
The Ecumenical Council of Chalcedon (AD 451) Held twenty years after the third General Council, this was to answer the Eutychian or Monophysite heresy and affirm the doctrine of two natures in Christ. This followed as a result of the growing controversy among the early theologians who were being led into error by a confused idea of the one divine person being both God and man or that there are two natures, human and divine, in the one person of the Word.
The Second Ecumenical Council of Constantinople (AD 553) This Council is sometimes referred to as the Council of the Three Chapters because its chief work was to condemn the writings and teaching of Theodore of Mopsuestia, the erroneous portions in the writings of Theodoret, and the letters of Ibas. It reaffirmed the dogmas stated by the third and forth General Councils.
The Third Ecumenical Council of Constantinople (AD 680-681) This Council gave the definition of two wills in Christ as the true teaching against the Monothelite heresy which claimed only one will.
The Second Ecumenical Council of Nicaea (AD 757) Here was defined the veneration due to holy images, that we give honor only to those they represent and not to the image itself as such; it presented the answer to the image breakers or iconoclasts. It also gave twenty-two canons regarding the clergy.