13 But other writings of Dionysius have to be considered also. Their history.
The following is the occasion of his writing the other letters. The Bishop Dionysius having heard of the affairs in Pentapolis and having written, in zeal for religion, as I said above, his letter to Euphranor and Ammonius against the heresy of Sabellius, some of the brethren belonging to the Church, of right opinions, but without asking him, so as to learn from himself how he had written, went up to Rome; and they spoke against him in the presence of his namesake Dionysius the Bishop of Rome. And he, upon hearing it, wrote simultaneously against the partisans of Sabellius and against those who held the very opinions for uttering which Arius was cast out of the Church; calling it an equal and opposite impiety to hold with Sabellius, or with those who say that the Word of God is a thing made and formed and originated. And he wrote also to Dionysius to inform him of what they had said about him. And the latter straightway wrote back, and inscribed his books 'a Refutation and a Defence.' Here mark the detestable gang of the adversaries of Christ, and how they themselves have stirred up their disgrace against themselves. For Dionysius, Bishop of Rome, having written also against those who said that the Son of God was a creature and a created thing, it is manifest that not now for the first time but from of old the heresy of the Arian adversaries of Christ has been anathematised by all. And Dionysius, Bishop of Alexandria, making his defence concerning the letter he had written, appears in his turn as neither thinking as they allege, nor having held the Arian error at all.
Source: De Sententia Dionysii (New Advent)