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Legacy

The historian A.H.M. Jones observed that "It is perhaps Diocletian's greatest achievement that he reigned twenty-one years and then abdicated voluntarily, and spent the remaining years of his life in peaceful retirement." Diocletian was one of the few emperors of the third and fourth centuries to die naturally, and the first in the history of the empire to retire voluntarily. Once he retired, his tetrarchic system collapsed. Without the guiding hand of Diocletian, the empire fell into civil wars. Stability emerged after the defeat of Licinius by Constantine in 324. Under the Christian Constantine, Diocletian was maligned. Constantine's rule, however, demonstrated the benefits of Diocletian's achievements and the autocratic principle he represented: the borders remained secure, in spite of Constantine's large expenditure of forces during his civil wars; the bureaucratic transformation of the Roman government was completed; and Constantine took Diocletian's court ceremonies and made them even more extravagant.

Constantine ignored those aspects of Diocletian's reign that did not suit him. Diocletian's policy of preserving a stable silver coinage was abandoned, and the gold solidus became the empire's primary currency instead. Diocletian's persecution of Christians was repudiated and changed to a policy of toleration and then favoritism. Christianity eventually became the official religion in 380. Most importantly, Diocletian's tax system and administrative reforms lasted, with some modifications, until the advent of the Muslims in the 630s. The combination of state autocracy and state religion was instilled in much of Europe, particularly in the lands which adopted Orthodox Christianity.

The Era of Martyrs (Latin: anno martyrum or AM), also known as the Diocletian era (Latin: anno Diocletiani), is a method of numbering years used by the Church of Alexandria beginning in the 4th century anno Domini and by the Coptic Orthodox Church of Alexandria from the 5th century to the present. In this system of counting, the beginning of Diocletian's reign in 284 was used as the epoch, making Diocletian's first year in power into the Year 1 of that calendar. Western Christians were aware of this count but did not use it; Dionysius Exiguus replaced the anno Diocletiani era with his anno Domini era because he did not wish to continue the memory of a tyrant who persecuted Christians.

Dukljan, a major villain in Serbian mythology who is presented as the adversary of God, is considered to be a mythological reflection of the historical Diocletian.

The Talmud includes several semi-legendary accounts of Diocletian. One of them recounts that Diocletian was originally a swineherd, and that in this part of his life, he was teased and abused by young Jews. When he became the Emperor he called up the leaders of the Jews, who were fearful, saying "We have teased Diocletian the Swineherd but we respect Diocletian the Emperor" – to which Diocletian responded, "You must show respect even to the smallest and lowest of the Romans, because you can never know which one of us will rise to greatness."