Pope Dionysius of Alexandria, named "the Great," was the Pope of Alexandria from 248 until his death on November 17, 265 after seventeen years as a bishop.
He is the first Pope to hold the title "the Great" (even before a Bishop of Rome).
We have information on Dionysius because during his lifetime, Dionysius writes many correspondence letters.
Only one original letter survives to this day; the remaining letters are found rewritten in the works of Eusebius.