19 Inconsistency of the Arian appeal to Dionysius.
Who that hears this will not set down as mad those who suspect Dionysius of holding with Arius? For lo! In these words, by arguments based on truth, he tramples upon his entire heresy. For by the simile of the Brightness he destroys the statements that 'He was not before He was begotten,' and 'There was a time when He was not,' as also by saying that His Father was never without issue. But their allegation that He was made 'of nothing' he destroys by saying that the Word was like a river from a well, and a shoot from a stock, and a child from a parent, and Light from Light, and Life from Life. And their barring off and separating the Word from God, he overthrows by saying that the Triad is without division and without diminution gathered together into the Monad. While their statement that the Son has no part in the Father's essence, he unequivocally tramples down by saying that the Son is of one essence with the Father. Wherein one must wonder at the impudence of the irreligious persons. How can they, when Dionysius whom they claim as their partisan says that the Son is of one essence, themselves go about buzzing like gnats with the complaint that the Synod was wrong in writing 'of one essence?' For if Dionysius is a friend of theirs, let them not deny what their partisan holds. But if they think that the expression was wrongly used, how can they reiterate that Dionysius, who used it, held with them? The more so as he does not appear to have written these things merely by the way, but having previously written other letters, he convicts of falsehood those who had charged him with not saying that the Son was of one Essence with the Father, while he refutes those who thought that he said that the Word was originated, showing that he did not hold what they supposed, but even if he had used the expressions, he had done so merely in order to show that it was the Son, not the Father, who had put on the originated, formed, created body; for which reason the Son also is said to have been originated, created, and formed.
Source: De Sententia Dionysii (New Advent)