1. In the third epistle on baptism which this same Dionysius wrote to Philemon, the Roman presbyter, he relates the following: But I examined the works and traditions of the heretics, defiling my mind for a little time with their abominable opinions, but receiving this benefit from them, that I refuted them by myself, and detested them all the more. 2. And when a certain brother among the presbyters restrained me, fearing that I should be carried away with the filth of their wickedness (for it would defile my soul)—in which also, as I perceived, he spoke the truth,— a vision sent from God came and strengthened me. 3. And the word which came to me commanded me, saying distinctly, "Read everything which you can take in hand, for you are able to correct and prove all; and this has been to you from the beginning the cause of your faith." I received the vision as agreeing with the apostolic word, which says to them that are stronger, "Be skillful money-changers." 4. Then after saying some things concerning all the heresies he adds: I received this rule and ordinance from our blessed father, Heraclas. For those who came over from heresies, although they had apostatized from the Church—or rather had not apostatized, but seemed to meet with them, yet were charged with resorting to some false teacher—when he had expelled them from the Church he did not receive them back, though they entreated for it, until they had publicly reported all things which they had heard from their adversaries; but then he received them without requiring of them another baptism. For they had formerly received the Holy Spirit from him. 5. Again, after treating the question thoroughly, he adds: I have learned also that this is not a novel practice introduced in Africa alone, but that even long ago in the times of the bishops before us this opinion has been adopted in the most populous churches, and in synods of the brethren in Iconium and Synnada, and by many others. To overturn their counsels and throw them into strife and contention, I cannot endure. For it is said, "You shall not remove your neighbor's landmark, which your fathers have set." (Deut 19:14) 6. His fourth epistle on baptism was written to Dionysius of Rome, who was then a presbyter, but not long after received the episcopate of that church. It is evident from what is stated of him by Dionysius of Alexandria, that he also was a learned and admirable man. [...]

 

(trans. A. Cushman McGiffert;  http://www.newadvent.org/fathers/250106.htm)