The Epistles which Dionysius wrote on this Subject.
Church History (Historia Ecclesiastica) · Eusebius of Caesarea
EusHE 9.5.1
Dionysius, therefore, having communicated with him extensively on this question by letter, finally showed him that since the persecution had abated, the churches everywhere had rejected the novelty of Novatus, and were at peace among themselves. He writes as follows: