De Synodis
33 Further the whole of the above statement has drawn no distinction whatever between the essence and nature of the Father and the Son. For when it is said, God of God, whole God of whole God, there is no room for doubting that whole God is born of whole God. For the nature of God who is of God admits of no difference, and as whole God of whole God He is in all in which the Father is. One of One excludes the passions of a human birth and conception, so that since He is One of One, He comes from no other source, nor is different nor alien, for He is One of One, perfect God of perfect God. Except in having a cause of its origin His birth does not differ from the birthless nature since the perfection of both Persons is the same. King of King. A power that is expressed by one and the same title allows no dissimilarity of power. Lord of Lord. In 'Lord' also the lordship is equal: there can be no difference where domination is confessed of both without diversity. But plainest of all is the statement appended after several others, unable to change or alter, the unvarying image of the Godhead and essence and might and glory. For as God of God, whole God of whole God, One of One, perfect God of perfect God, King of King and Lord of Lord, since in all that glory and nature of Godhead in which the Father ever abides, the Son born of Him also subsists; He derives this also from the Father's substance that He is unable to change. For in His birth that nature from which He is born is not changed; but the Son has maintained a changeless essence since His origin is in a changeless nature. For though He is an image, yet the image cannot alter, since in Him was born the image of the Father's essence, and there could not be in Him a change of nature caused by any unlikeness to the Father's essence from which He was begotten. Now when we are taught that He was brought into being as the first of all creation, and He is Himself said to have always been in the beginning with God as God the Word, the fact that He was brought into being shows that He was born, and the fact that He always was, shows that He is not separated from the Father by time. Therefore this Council by dividing the three substances, which it did to exclude a monad God with a threefold title, did not introduce any separation of substance between the Father and the Son. The whole exposition of faith makes no distinction between Father and Son, the Unborn and the Only-begotten, in time, or name, or essence, or dignity, or domination. But our common conscience demands that we should gain a knowledge of the other creeds of the same Eastern bishops, composed at different times and places, that by the study of many confessions we may understand the sincerity of their faith.
The Creed according to the Council of the East.
34. We, the holy synod met in Sardica from different provinces of the East, namely, Thebais, Egypt, Palestine, Arabia, Phœnicia, Cœle Syria, Mesopotamia, Cilicia, Cappadocia, Pontus, Paphlagonia, Galatia, Bithynia and Hellespont, from Asia, namely, the two provinces of Phrygia, Pisidia, the islands of the Cyclades, Pamphylia, Caria, Lydia, from Europe, namely, Thrace, Hæmimontus, Mœsia, and the two provinces of Pannonia, have set forth this creed.
We believe in one God, the Father Almighty, Creator and Maker of all things, from whom all fatherhood in heaven and earth is named:
And we believe in His Only-begotten Son our Lord Jesus Christ, who before all ages was begotten of the Father, God of God, Light of Light, through whom were made all things which are in heaven and earth, visible and invisible: who is the Word and Wisdom and Might and Life and true Light: and who in the last days for our sake was incarnate, and was born of the holy Virgin, who was crucified and dead and buried, And rose from the dead on the third day, And was received into heaven, And sits on the right hand of the Father, And shall come to judge the quick and the dead and to give to every man according to his works: Whose kingdom remains without end for ever and ever. For He sits on the right hand of the Father not only in this age, but also in the age to come.
We believe also in the Holy Ghost, that is, the Paraclete, whom according to His promise He sent to His apostles after His return into the heavens to teach them and to bring all things to their remembrance, through whom also the souls of them that believe sincerely in Him are sanctified.
“But those who say that the Son of God is sprung from things non-existent or from another substance and not from God, and that there was a time or age when He was not, the holy Catholic Church holds them as aliens. Likewise also those who say that there are three Gods, or that Christ is not God and that before the ages He was neither Christ nor Son of God, or that He Himself is the Father and the Son and the Holy Ghost, or that the Son is incapable of birth; or that the Father begot the Son without purpose or will: the holy Catholic Church anathematizes.”
35. In the exposition of this creed, concise but complete definitions have been employed. For in condemning those who said that the Son sprang from things non-existent, it attributed to Him a source which had no beginning but continues perpetually. And lest this source from which He drew His permanent birth should be understood to be any other substance than that of God, it also declares to be blasphemers those who said that the Son was born of some other substance and not of God. And so since He does not draw His subsistence from nothing, or spring from any other source than God, it cannot be doubted that He was born with those qualities which are God's; since the Only-begotten essence of the Son is generated neither from things which are non-existent nor from any other substance than the birthless and eternal substance of the Father. But the creed also rejects intervals of times or ages: on the assumption that He who does not differ in nature cannot be separable by time.
36. On every side, where anxiety might be felt, approach is barred to the arguments of heretics lest it should be declared that there is any difference in the Son. For those are anathematized who say that there are three Gods: because according to God's true nature His substance does not admit a number of applications of the title, except as it is given to individual men and angels in recognition of their merit, though the substance of their nature and that of God is different. In that sense there are consequently many gods. Furthermore in the nature of God, God is one, yet in such a way that the Son also is God, because in Him there is not a different nature: and since He is God of God, both must be God, and since there is no difference of kind between them there is no distinction in their essence. A number of titular Gods is rejected; because there is no diversity in the quality of the divine nature. Since therefore he is anathema who says there are many Gods and he is anathema who denies that the Son is God; it is fully shown that the fact that each has one and the same name arises from the real character of the similar substance in each: since in confessing the Unborn God the Father, and the Only-begotten God the Son, with no dissimilarity of essence between them, each is called God, yet God must be believed and be declared to be one. So by the diligent and watchful care of the bishops the creed guards the similarity of the nature begotten and the nature begetting, confirming it by the application of one name.
Source: On the Councils, or the Faith of the Easterns (New Advent)