Dionysius was a respected presbyter under Sixtus II; Dionysius of Alexandria wrote to him regarding the administration of baptism by heretics Euseb., HE VII, 5,6. Under Gallienus, who had restored to the Christians the churches expropriated by Valerian, Dionysius, himself now a bishop, reorganized the Roman community. At the request of Libyan presbyters, he called a synod which condemned both Sabellius and the tendency called Marcionite to break the divine monarchy into three hypostases; he communicated these decisions to Dionysius of Alexandria, accused by the presbyters of tritheism DS 112-115. The Alexandrian bishop justified his trinitarian theology by further explaining the possible use of o`moou,sioj, which he had previously rejected see Athan., Sent. Dion.. According to Basil Ep. 70, Dionysius helped the communities of Cappadocia, devastated by foreign invaders. He never received the letter of the Synod of Antioch that had condemned Paul of Samosata Euseb., HE VII, 7. Not unjustly is he considered one of the most important popes of the 3rd c.