De Synodis
28 Here, beloved brethren, is the entire creed which was published by some Easterns, few in proportion to the whole number of bishops, and which first saw light at the very times when you repelled the introduction of this heresy. The reason for its promulgation was the fact that they were bidden to say nothing of the ὁμοούσιον. But even in former times, through the urgency of these numerous causes, it was necessary at different occasions to compose other creeds, the character of which will be understood from their wording. For when you are fully aware of the results, it will be easier for us to bring to a full consummation, such as religion and unity demand, the argument in which we are interested.
An exposition of the faith of the Church made at the Council held on the occasion of the Dedication of the church at Antioch by ninety-seven bishops there present, because of suspicions felt as to the orthodoxy of a certain bishop
29. We believe in accordance with evangelical and apostolic tradition in one God the Father Almighty, the Creator, Maker and Disposer of all things that are, and from whom are all things.
And in one Lord Jesus Christ, His Only-begotten Son, God through whom are all things, who was begotten of the Father, God of God, whole God of whole God, One of One, perfect God of perfect God, King of King, Lord of Lord, the Word, the Wisdom, the Life, true Light, true Way, the Resurrection, the Shepherd, the Gate, unable to change or alter, the unvarying image of the essence and might and glory of the Godhead, the first-born of all creation, who always was in the beginning with God, the Word of God, according to what is said in the Gospel, and the Word was God, through whom all things were made, and in whom all things subsist, who in the last days came down from above, and was born of a virgin according to the Scriptures, and was made the Lamb, the Mediator between God and man, the Apostle of our faith, and leader of life. For He said, I came down from heaven, not to do My own will, but the will of Him that sent me. Who suffered and rose again for us on the third day, and ascended into heaven, and sits on the right hand of the Father, and is to come again with glory to judge the quick and the dead.
And in the Holy Ghost, who was given to them that believe, to comfort, sanctify and perfect, even as our Lord Jesus Christ ordained His disciples, saying, Go, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost, manifestly, that is, of a Father who is truly Father, and clearly of a Son who is truly Son, and a Holy Ghost who is truly a Holy Ghost, these words not being set forth idly and without meaning, but carefully signifying the Person, and order, and glory of each of those who are named, to teach us that they are three Persons, but in agreement one.
30. “Having therefore held this faith from the beginning, and being resolved to hold it to the end in the sight of God and Christ, we say anathema to every heretical and perverted sect, and if any man teaches contrary to the wholesome and right faith of the Scriptures, saying that there is or was time, or space, or age before the Son was begotten, let him be anathema. And if any one say that the Son is a formation like one of the things that are formed, or a birth resembling other births, or a creature like the creatures, and not as the divine Scriptures have affirmed in each passage aforesaid, or teaches or proclaims as the Gospel anything else than what we have received: let him be anathema. For all those things which were written in the divine Scriptures by Prophets and by Apostles we believe and follow truly and with fear.”
31. Perhaps this creed has not spoken expressly enough of the identical similarity of the Father and the Son, especially in concluding that the names Father, Son and Holy Ghost referred to the Person and order and glory of each of those who are named to teach us that they are three Persons, but in agreement one.
32. But in the first place we must remember that the bishops did not assemble at Antioch to oppose the heresy which has dared to declare that the substance of the Son is unlike that of the Father, but to oppose that which, in spite of the Council of Nicæa, presumed to attribute the three names to the Father. Of this we will treat in its proper place. I recollect that at the beginning of my argument I besought the patience and forbearance of my readers and hearers until the completion of my letter, lest any one should rashly rise to judge me before he was acquainted with the entire argument. I ask it again. This assembly of the saints wished to strike a blow at that impiety which by a mere counting of names evades the truth as to the Father and the Son and the Holy Ghost; which represents that there is no personal cause for each name, and by a false use of these names makes the triple nomenclature imply only one Person, so that the Father alone could be also called both Holy Ghost and Son. Consequently they declared there were three substances, meaning three subsistent Persons, and not thereby introducing any dissimilarity of essence to separate the substance of Father and Son. For the words to teach us that they are three in substance, but in agreement one, are free from objection, because as the Spirit is also named, and He is the Paraclete, it is more fitting that a unity of agreement should be asserted than a unity of essence based on likeness of substance.
Source: On the Councils, or the Faith of the Easterns (New Advent)