Diocletian was born in 240 BC and grew up during the Third Century Crisis, a period when the Roman empire was very unstable and on the verge of collapse because of power struggles and ineffective rulers. Young Diocletian joined the military and rose through the top ranks.
Rise of Diocletian
It was around 282 BC when a division of Roman soldiers (i.e. a legion) had proclaimed their commander, Carus, as the new emperor. Emperor Carus liked Diocletian and elevated him to the highest ranks.
Emperor Carus only ruled for about a year and (like was explained earlier) was killed by a lightening. His sons – Numerian and Carinus – succeeded him. They only ruled for a very short period of time. Numerian died of an illness (most likely a result of being poisoned). Carinus was in conflict with Diocletian. A battle between Carinus and Diocletian ensued. Carinus was unpopular and most of his men defected to Diocletian’s side. Carinus got killed by his own soldiers and the armies of both the western and eastern empire rallied behind Diocletian and proclaimed him emperor.
Emperor Diocletian became determined to completely put to an end the instability that characterized the empire. There were two main challenges for Diocletian to overcome: the internal struggles for power in the empire, and the horde of barbarian tribes (especially from the eastern side of the empire) which kept attempting to attack and destroy Rome. Diocletian was set to protect the empire from collapsing through various reforms.
Diocletian established himself as a powerful dictator who could never be challenged. He de-politicized the army and ensured total allegiance to his authority.
Noticing that the empire faced immense danger of Persian invasions on its eastern borders, he realized that the empire was too vast to be governed from one central point, Rome. In 285 AD he divided the empire into two halves – the western half to be ruled by a lesser emperor (addressed by the title Caesar) and himself (a senior emperor addressed by the title Augustus) to rule over the eastern half of the empire. With these developments, Rome ceased to be the capital of the empire; the Western Empire was ruled from Milan ( a city in northern Italy), and the Eastern Empire from Nicomedia (now Izmir in Turkey)
In 293 AD another change was made: each half of the empire would have both an Augustus and a Caesar so that the whole empire now had four rulers. This system became known as the Tetrarchy (i.e. “government by four people”). During the Tetrarchy, Emperor Diocletian was Augustus of the eastern half of the empire and watched over Thrace, Asia, and Egypt. Galerius as Ceaser under Diocletian watched over Illyria, the Danubian provinces, and Achaea. Emperor Maximian was Augustus of the western empire and watched over Italy, Sicily, and Africa. Constantius was Ceaser serving under Maximian and ruled over Gaul, Spain, and Britain.
The tetrarchy was an effective system of administration which made the presence of imperial authority felt throughout the empire.
Now, Diocletian is much known for two things – first, the great organization (administration) he established in the empire and second, the terrible, worst, and last persecution he caused upon Christians.
Beginning of Christian Persecution
Sometime in the year 299 AD there was an important ceremony of sacrifices in Antioch which was officiated by emperors. Performing the ritual were haruspices. Haruspices were diviners in the religion of Rome and used to read entrails of sacrificed animals in order to predict the future. At the special event of 299 AD they failed to read the omen of the entrails and accused the Christians to be causers of the failure. The emperors ordered people to offer sacrifices to purify the place. The order was extended to all men serving in the army; everyone was required to offer a sacrifice or he would be dismissed.
Such orders to offer sacrifices always put Christians in trouble. Christians soldiers would refuse to sacrifice, get accused of disobedience, and were killed. This time Emperor Diocletian decided to recall every Christian from serving in the army. However, Caesar Galerius was bent on influencing Diocletian to further exterminate Christians. Diocletian was at first reluctant to enforce such an extreme response to Christian’s exclusive lifestyles, but he later turned into an ugly monster that devised every possible tool and way to inflict pain on Christians.
After returning from Antioch, Diocletian was angry with Deacon Romanus, a Christian leader who had been vocal against pagan sacrifices. He ordered that his tongue be pulled from his mouth. John Foxe tells the details of the torture of this saint: “he was scourged, put to the rack, his body torn with hooks, his flesh cut with knives, his face scarified, his teeth beaten from their sockets, and his hair plucked up by the roots. Soon after he was ordered to be strangled, November 17, A.D. 303.”
On 24th February, 303 AD Diocletian issued The Edict Against the Christians which ordered the destruction of Scriptures and worship places of Christians, and further prohibited them from meeting for prayers.
A few days after the edict had been issued there was an inferno which gutted a portion of the imperial palace. Galerius blamed it on Christians. Even after an investigation into the matter did not find the causers of the fire, Christians had to bear the consequences: A Christian by the name Peter Cubicularius was stripped of his clothes and scourged. Salt and vinegar were poured into his wounds. After this excruciating experience he was slowly boiled over an open flame. A terrible persecution thereafter ensued in which “no distinction was made of age or sex” writes John Foxe. “the name of Christian was so obnoxious to the pagans that all indiscriminately fell sacrifices to their opinions. Many houses were set on fire, and whole Christian families perished in the flames; and others had stones fastened about their necks, and being tied together were driven into the sea. The persecution became general in all the Roman provinces, but more particularly in the east; and as it lasted ten years, it is impossible to ascertain the numbers martyred, or to enumerate the various modes of martyrdom.”
Cause of the Diocletian Persecution?
As to why the Diocletian Persecution occurred, Jean Cousin in Encyclopaedia Britannica wrote:
The reasons for this persecution are uncertain, but various explanations have been advanced: the possible influence of Galerius, a fanatic follower of the traditional Roman religion; the desire to restore complete unity, without tolerance of a foreign cult that was seen as separatist and of individuals who were forming a kind of state within the state; the influence of anti-Christian philosophers such as Porphyry and governors such as Hierocles on the scholarly class and on the imperial court; the fear of an alienation of rebellious armies from emperor worship; or perhaps the disturbances provoked by the Christians themselves, who were agitated by doctrinal controversies. At any rate, some or all of these factors led Diocletian to publish the four edicts of 303–304, promising all the while that he would not spill blood. His vow went unheeded, however, and the persecutions spread through the empire with an extreme violence that did not succeed in annihilating Christianity but caused the faith of the martyrs to blaze forth instead (Cousin, 2017).
But our Lord had clearly foretold saying, “Remember the word that I said unto you, The servant is not greater than his lord. If they have persecuted me, they will also persecute you; if they have kept my saying, they will keep yours also. But all these things will they do unto you for my name’s sake, because they know not him that sent me” (Joh.15:20-21). And as history has repeatedly shown, no amount of suppression or persecution can break the spirit of a people who believe. The flames of fire, or cruelty of a sword, can never vanquish the faith of a soul that is sincerely devoted to a cause. Amidst all the trials and cruelty the Christians suffered, God had not abandoned them. Satan, the master of political systems of this world, sought to destroy the saints but faith in God’s Word and the joy of the Lord gave them strength to press on. Prophecies, like the ones which came through the aging and last surviving apostle of Jesus, John, were a source of encouragement: “Fear none of those things which thou shalt suffer: behold, the devil shall cast some of you into prison, that ye may be tried; and ye shall have tribulation ten days: be thou faithful unto death, and I will give thee a crown of life”(Rev.2:10).
Indeed, when historians are listing failures of Diocletian, one thing that cannot go unmentioned is that despite his campaign against the Christians having been the largest and bloodiest, it not only failed to eliminate Christianity but was shortly followed by its increased popularity, and only about a decade later became the state religion of the empire. It was such a sharp and unpredictable swing in history.
 The first emperor of Rome was Augustus. After his death his name became the official title of emperors.