Dionysius was a pupil of Origen in the great catechetical school in Alexandria, and then headed that school himself for fourteen years. In 247 he was made bishop of Alexandria. Persecution soon after broke out, and he directed his church from hiding for several years, until the death of the Emperor Decian in 251. In the period of peace that followed, Dionysius was a zealous supporter of lenient treatment for the lapsed (those who had renounced the faith under pressure, and now wished to be received back into the Church). His seventeen years as a bishop were turbulent ones, during which Alexandria suffered civil strife, plague, and famine, and the church was subject to repeated persecutions. He died on 17 November 265, but his commemoration is here transferred to the first open day. (What constitutes an "open" day depends, of course, on whom else one is commemorating.)
Dionysius was a scholar, a keen student of pagan as well as Christian learning.
Faced with the rise of a heretical opinion in his diocese, he proceeded, not by excommunication, but by challenging his opponents to a public debate, with the thirty of them on one side and himself on the other. The debate lasted three days, by the end of which time he had talked all thirty of his opponents round to his side.
Only a few of his considerable writings have survived. One detail that I have encountered in a Bible commentary is his suggestion that the book of Revelation had a different author from the Gospel of John, a view which he based on differences in the style of writing and other internal evidence. (The reader is reminded that the writer of the book of Revelation tells us that his name is John, but not that he is the Apostle of that name, so that Dionysius cannot be said to be contradicting the Bible.)
O God, who hast enlightened thy Church by the teaching of thy Servant Dionysius: Enrich us evermore with thy heavenly grace, and raise up faithful witnesses who by their life and doctrine will set forth the truth of thy salvation; through Jesus Christ our Lord, who liveth and reigneth with thee and the Holy Spirit, one God, now and ever.
O God, who have enlightened your Church by the teaching of your Servant Dionysius: Enrich us evermore with your heavenly grace, and raise up faithful witnesses who by their life and doctrine will set forth the truth of your salvation; through Jesus Christ our Lord, who lives and reigns with you and the Holy Spirit, one God, now and ever.