De Synodis
63 You must not be surprised, dear brethren, that so many creeds have recently been written. The frenzy of heretics makes it necessary. The danger of the Eastern Churches is so great that it is rare to find either priest or layman that belongs to this faith, of the orthodoxy of which you may judge. Certain individuals have acted so wrongly as to support the side of evil, and the strength of the wicked has been increased by the exile of some of the bishops, the cause of which you are acquainted with. I am not speaking about distant events or writing down incidents of which I know nothing: I have heard and seen the faults which we now have to combat. They are not laymen but bishops who are guilty. Except the bishop Eleusius and his few comrades, the greater part of the ten provinces of Asia, in which I am now staying, really know not God. Would that they knew nothing about Him, for their ignorance would meet with a readier pardon than their detraction. These faithful bishops do not keep silence in their pain. They seek for the unity of that faith of which others have long since robbed them. The necessity of a united exposition of that faith was first felt when Hosius forgot his former deeds and words, and a fresh yet festering heresy broke out at Sirmium. Of Hosius I say nothing, I leave his conduct in the background lest man's judgment should forget what once he was. But everywhere there are scandals, schisms and treacheries. Hence some of those who had formerly written one creed were compelled to sign another. I make no complaint against these long-suffering Eastern bishops, it was enough that they gave at least a compulsory assent to the faith after they had once been willing to blaspheme. I think it a subject of congratulation that a single penitent should be found among such obstinate, blaspheming and heretical bishops. But, brethren, you enjoy happiness and glory in the Lord, who meanwhile retain and conscientiously confess the whole apostolic faith, and have hitherto been ignorant of written creeds. You have not needed the letter, for you abounded in the spirit. You required not the office of a hand to write what you believed in your hearts and professed unto salvation. It was unnecessary for you to read as bishops what you held when new-born converts. But necessity has introduced the custom of expounding creeds and signing expositions. Where the conscience is in danger we must use the letter. Nor is it wrong to write what it is wholesome to confess.
64. Kept always from guile by the gift of the Holy Spirit, we confess and write of our own will that there are not two Gods but one God; nor do we therefore deny that the Son of God is also God; for He is God of God. We deny that there are two incapable of birth, because God is one through the prerogative of being incapable of birth; nor does it follow that the Unbegotten is not God, for His source is the Unborn substance. There is not one subsistent Person, but a similar substance in both Persons. There is not one name of God applied to dissimilar natures, but a wholly similar essence belonging to one name and nature. One is not superior to the other on account of the kind of His substance, but one is subject to the other because born of the other. The Father is greater because He is Father, the Son is not the less because He is Son. The difference is one of the meaning of a name and not of a nature. We confess that the Father is not affected by time, but do not deny that the Son is equally eternal. We assert that the Father is in the Son because the Son has nothing in Himself unlike the Father: we confess that the Son is in the Father because the existence of the Son is not from any other source. We recognize that their nature is mutual and similar because equal: we do not think them to be one Person because they are one: we declare that they are through the similarity of an identical nature one, in such a way that they nevertheless are not one Person.
65. I have expounded, beloved brethren, my belief in our common faith so far as our wonted human speech permitted and the Lord, whom I have ever besought, as He is my witness, has given me power. If I have said too little, nay, if I have said almost nothing, I ask you to remember that it is not belief but words that are lacking. Perhaps I shall thereby prove that my human nature, though not my will, is weak: and I pardon my human nature if it cannot speak as it would of God, for it is enough for its salvation to have believed the things of God.
66. Since your faith and mine, so far as I am conscious, is in no danger before God, and I have shown you, as you wished, the creeds that have been set forth by the Eastern bishops (though I repeat that they were few in number, for, considering how numerous the Eastern Churches are, that faith is held by few), I have also declared my own convictions about divine things, according to the doctrine of the apostles. It remains for you to investigate without suspicion the points that mislead the unguarded temper of our simple minds, for there is now no opportunity left of hearing. And although I shall no longer fear that sentence will not be passed upon me in accordance with the whole exposition of the creed, I ask you to allow me to express a wish that I may not have the sentence passed until the exposition is actually completed.
67. Many of us, beloved brethren, declare the substance of the Father and the Son to be one in such a spirit that I consider the statement to be quite as much wrong as right. The expression contains both a conscientious conviction and the opportunity for delusion. If we assert the one substance, understanding it to mean the likeness of natural qualities and such a likeness as includes not only the species but the genus, we assert it in a truly religious spirit, provided we believe that the one substance signifies such a similitude of qualities that the unity is not the unity of a monad but of equals. By equality I mean exact similarity so that the likeness may be called an equality, provided that the equality imply unity because it implies an equal pair, and that the unity which implies an equal pair be not wrested to mean a single Person. Therefore the one substance will be asserted piously if it does not abolish the subsistent personality or divide the one substance into two, for their substance by the true character of the Son's birth and by their natural likeness is so free from difference that it is called one.
Source: On the Councils, or the Faith of the Easterns (New Advent)