Diocletian stabilised Rome in the third century by establishing the Tetrarchy. His system of four rulers, Senior (Augustus) and Junior (Caesar) in both Eastern and Western halves of the Empire allowed Rome a respite from internal conflict.
Almost as soon as he died the stability of his system began to fray.
Diocletian was also very set against Christianity and was responsible for some of the worst persecutions of Christians in the Roman Empire.
Constantine was not a Christian himself, but his Mother Helena certainly was. We must evaluate her role in the preparations for the battle of Milvian Bridge on Oct 28th 312 CE. On the night before the battle Constantine instructed his troops to mark their shields with the Chi Rho symbol, the first two letters in the Greek name for Christ.
According to the Christian Church this was because Constantine had a vision from God. My interpretation is that he probably had a visitation from the Christians of Rome. Many of his troops were already Christian converts who could not be open about their faith in the Diocletian era.
Many of his rival’s troops were also Christian. We could question how many of the troops led by Maxentius refused to engage once they encountered the Chi-Rho banner, the promise of freedom to practice their faith.
I believe that Constantine, through the negotiations of his Mother, was able to swing the battle in his favour by declaring his “acceptance” of Christianity.
Constantine won the day and went on to become Constantine “The Great”, founder of the Byzantine Empire. The system set up by him endured for another 900 years.
Sailing to Byzantium; by WB Yeats
That is no country for old men. The young
In one another’s arms, birds in the trees,
—Those dying generations—at their song,
The salmon-falls, the mackerel-crowded seas,
Fish, flesh, or fowl, commend all summer long
Whatever is begotten, born, and dies.
Caught in that sensual music all neglect
Monuments of unageing intellect.
An aged man is but a paltry thing,
A tattered coat upon a stick, unless
Soul clap its hands and sing, and louder sing
For every tatter in its mortal dress,
Nor is there singing school but studying
Monuments of its own magnificence;
And therefore I have sailed the seas and come
To the holy city of Byzantium.
O sages standing in God’s holy fire
As in the gold mosaic of a wall,
Come from the holy fire, perne in a gyre,
And be the singing-masters of my soul.
Consume my heart away; sick with desire
And fastened to a dying animal
It knows not what it is; and gather me
Into the artifice of eternity.
Once out of nature I shall never take
My bodily form from any natural thing,
But such a form as Grecian goldsmiths make
Of hammered gold and gold enamelling
To keep a drowsy Emperor awake;
Or set upon a golden bough to sing
To lords and ladies of Byzantium
Of what is past, or passing, or to come.