5. Among the rest of the councils we know there are four venerable synods which embrace the whole faith in its chief heads, like the four Gospels or the four rivers of Paradise.
6. Of these the first, the Nicene synod of 318 bishops, was held when Constantine was emperor. In it the blasphemy of the Arian perfidy was condemned, which the same Arius gave utterance to concerning the inequality of the holy Trinity. The same holy synod in the creed defined God the son as consubstantial with God, the father.
7. The second synod of 150 fathers gathered at Constantinople under Theodosius the elder, and condemning Macedonius, who denied that the Holy Spirit was God, proved that the Holy Spirit was consubstantial with the Father and the Son, giving the form of the creed which the whole confession, Greek and Latin, preaches in the churches.
8. The third synod, the first of Ephesus, of 200 bishops, was held under Theodosius II, and it condemned with a just anathema Nestorius, who asserted that there were two persons in Christ, and showed that the one person of the Lord Jesus Christ was immanent in the two natures.
9. The fourth synod of 630 priests was held at Chalcedon under Martianus, and it condemned by the unanimous vote of the fathers Euthyches, abbot of Constantinople, who asserted that the nature of the Word of God and of flesh was one, and his defender, Dioscorus, bishop of Alexandria, and Nestorius himself a second time, along with the remaining heretics, the same synod stating that Christ the Lord was so born of the virgin that we confess in him the substance both of the divine and of the human nature.
These four are the principal synods, stating most fully the doctrine of faith; and whatever councils there are which the holy Fathers, full of the spirit of God, have ratified, after the authority of these four, they continue established in all strength.
Chapter 17. The cycle of Easter.
10. After the completion of this [95-year cycle][313] a return must be made to the beginning. In ancient times the church used to celebrate Easter on the 14th of the moon at the same time as the Jews, whatever day it came on; this way of celebrating the holy Fathers forbade at the council of Nicaea, giving directions to make inquiry not only for the Easter moon and month, but also to observe the day of the resurrection of the Lord, and because of this they extended Easter from the 14th of the moon to the 21st, in order that the dies Dominicus might not be left out.
12. The eve of Easter is spent in watching because of the coming of our King and God, that the time of the resurrection may find us not sleeping but waking. And the reason for this night is a double one, either because he received life at that time when he suffered, or because he is to come for judgment at the same hour at which he arose.
13. And we celebrate Easter in such a way as not merely to call to memory the death and resurrection of Christ but also to consider the rest that is told about him with reference to its mystic meaning (ad sacramentorum significationem).