bishop of Alexandria, then the most important Eastern see, and a chief opponent of Sabellianism (q.v.).

A Christian convert, Dionysius studied in Alexandria at the catechetical school headed by Origen, whom in 231/232 he was elected to succeed. In 247/248 he became bishop of Alexandria. During the persecution (250–251) of Christians by the Roman emperor Decius, Dionysius fled to the Libyan Desert, and he was again exiled in the Valerian persecution (257–260).

On his return to Alexandria in about 260, Dionysius favoured readmitting penitent apostates to the church in opposition to those who wanted to exclude them permanently. Engaged in the bitter controversy over baptism performed by heretics, Dionysius did not insist on rebaptizing converts who had received heretical baptism, but he recognized the right of communities to rebaptize if they preferred. He denied that the Book of Revelation was written by St. John the Evangelist and denounced the Millenarians, who, basing their argument on a literal reading of Revelation, believed that after 1,000 years Jesus Christ would return and establish his kingdom on Earth.

Dionysius was especially noted for his attacks on the Sabellians, who accused him of separating the persons of the Trinity (tritheism) and other heresies. Protests were sent to Pope St. Dionysius (Dionysius, Saint) in Rome, who condemned those who denied any distinction between the persons of the Trinity and those who acknowledged three separate persons. Dionysius of Alexandria accepted the Pope's judgment and repudiated the Sabellians' charges, but he insisted that the Trinity consisted of three inseparable persons. His position has since been vindicated by the church.

Dionysius also wrote a treatise on nature against the atomism of the Greek philosopher Epicurus. Though highly esteemed and often cited by the leading Byzantine theologians, his works are known only from quotations, many of them extensive, preserved by Bishop Eusebius of Caesarea and other ecclesiastical writers.