Letter Against Sebellianism by Pope St. Dionysius

The following is an excerpt from a letter by Pope St. Dionysius about the Trinity. The excerpt is preserved in St. Athanasius' De Decretis Nicaenae Synodi Chapter 6. I've broken the text into paragraphs and numbered them to make the excerpt easier to read and cite. The ellipsis in paragraph 1 is used to indicate that St. Athanasius omitted something which I can't currently supply because I'm not aware of any source outside of St. Athanasius who has preserved this letter.

St. Athanasius may imply another significant feature of this letter: it apparently used the word "homoousios" and suggested that this word could be used as a mark of orthodoxy among bishops. There are several English translations of this word. The one used in the typical English translation of the Nicene Creed is consubstantial, but NewAdvent translates it "coessential." Pope Dionysius assembled a Roman council in part to compose the following letter, and St. Athanasius says that that council "made...mention of the Coessential" in order to assert the united nature of the Father and the Son. (De Synodis Part 3 Paragraph 45) The Nicene Creed adopted this word in the next century for the same purpose, which would make this letter an important precursor of the definition of the Council of Nicaea. If this is the letter where the Roman council used that word, however, the following excerpt does not appear to contain that part.

Letter Against Sebellianism by Pope St. Dionysius - ~260 A.D.

1. ... Next, I may reasonably turn to those who divide and cut to pieces and destroy that most sacred doctrine of the Church of God, the Divine Monarchy, making it as it were three powers and partitive subsistences and god-heads three.

2. I am told that some among you who are catechists and teachers of the Divine Word, take the lead in this tenet, who are diametrically opposed, so to speak, to Sabellius's opinions; for he blasphemously says that the Son is the Father, and the Father the Son, but they in some sort preach three Gods, as dividing the sacred Monad into three subsistences foreign to each other and utterly separate.

3. For it must needs be that with the God of the Universe, the Divine Word is united, and the Holy Ghost must repose and habitate in God; thus in one as in a summit, I mean the God of the Universe, must the Divine Triad be gathered up and brought together.

4. For it is the doctrine of the presumptuous Marcion, to sever and divide the Divine Monarchy into three origins—a devil's teaching, not that of Christ's true disciples and lovers of the Saviour's lessons. For they know well that a Triad is preached by divine Scripture, but that neither Old Testament nor New preaches three Gods.

5. Equally must one censure those who hold the Son to be a work, and consider that the Lord has come into being, as one of things which really came to be; whereas the divine oracles witness to a generation suitable to Him and becoming, but not to any fashioning or making.

6. A blasphemy then is it, not ordinary, but even the highest, to say that the Lord is in any sort a handiwork. For if He came to be Son, once He was not; but He was always, if (that is) He be in the Father, as He says Himself, and if the Christ be Word and Wisdom and Power (which, as you know, divine Scripture says), and these attributes be powers of God. If then the Son came into being, once these attributes were not; consequently there was a time, when God was without them; which is most absurd.

7. And why say more on these points to you, men full of the Spirit and well aware of the absurdities which come to view from saying that the Son is a work?

8. Not attending, as I consider, to this circumstance, the authors of this opinion have entirely missed the truth, in explaining, contrary to the sense of divine and prophetic Scripture in the passage, the words, 'The Lord created me a beginning of His ways unto His works.' [Proverbs 8:22]

9. For the sense of 'He created,' as you know, is not one, for we must understand 'He created' in this place, as 'He set over the works made by Him,' that is, 'made by the Son Himself.' And 'He created' here must not be taken for 'made,' for creating differs from making. 'Is not He your Father that has bought you? Has He not made you and created you?' [Deuteronomy 32:6] says Moses in his great song in Deuteronomy.

10. And one may say to them, O reckless men, is He a work, who is 'the First-born of every creature, who is born from the womb before the morning star,' who said, as Wisdom, 'Before all the hills He begets me?' [Proverbs 8:25]

11. And in many passages of the divine oracles is the Son said to have been generated, but nowhere to have come into being; which manifestly convicts those of misconception about the Lord's generation, who presume to call His divine and ineffable generation a making.

12. Neither then may we divide into three Godheads the wonderful and divine Monad; nor disparage with the name of 'work' the dignity and exceeding majesty of the Lord; but we must believe in God the Father Almighty, and in Christ Jesus His Son, and in the Holy Ghost, and hold that to the God of the universe the Word is united. For 'I,' says He, 'and the Father are one;' and, 'I in the Father and the Father in Me.'

13. For thus both the Divine Triad, and the holy preaching of the Monarchy, will be preserved.

[St. Athanasius does not appear to preserve the rest of the letter, if there is more.]

Source: http://www.newadvent.org/fathers/2809.htm