In the first place Constantine's "conversion' if that is the appropriate word does not seem to have been Christian at all but unabashedly pagan. He appears to have had some sort of vision, or numinous experience, in the precincts of a pagan temple to the Gallic Apollo, either in the Vosges or near Autun. According to a witness accompanying Constantine's army at the time, the vision was of the sun god the deity worshipped by certain cults under the name of "Sol Invictus', "the Invincible Sun'. There is evidence that Constantine, just before his vision, had been initiated into a Sol Invictus cult. In any case the Roman Senate, after the Battle of Milvian Bridge, erected a triumphal arch in the Colosseum. According to the inscription on this arch, Constantine's victory was won "through the prompting of the Deity'. But the Deity in question was not Jesus. It was Sol Invictus, the pagan sun god.
Contrary to tradition, Constantine did not make Christianity the official state religion of Rome. The state religion of Rome under Constantine was, in fact, pagan sun worship; and Constantine, all his life, acted as its chief priest. Indeed his reign was called a "sun emperorship" and Sol Invictus figured everywhere including the imperial banners and the coinage of the realm. The image of Constantine as a fervent convert to Christianity is clearly wrong. He himself was not even baptised until 337 when he lay on his deathbed and was apparently too weakened or too apathetic to protest.
Nor can he be credited with the Chi Rho monogram. An inscription bearing this monogram was found on a tomb at Pompeii, dating from two and a half centuries before. The cult of Sol Invictus was Syrian in origin and imposed by Roman emperors on their subjects a century before Constantine. Although it contained elements of Baal and Astarte worship, it was essentially monotheistic. In effect, it posited the sun god as the sum of all attributes of all other gods, and thus peacefully subsumed its potential rivals. Moreover, it conveniently harmonised with the cult of Mithras which was also prevalent in Rome and the empire at the time, and which also involved solar worship.
For Constantine the cult of Sol Invictus was, quite simply, expedient. His primary, indeed obsessive, objective was unity unity in politics, in religion and in territory. A cult, or state religion, that included all other cults within it obviously abetted this objective. And it was under the auspices of the Sol Invictus cult that Christianity consolidated its position. Christian orthodoxy had much in common with the cult of Sol Invictus; and thus the former was able to flourish unmolested under the taller's umbrella of tolerance. The cult of Sol Invictus, being essentially monotheistic, paved the way for the monotheism of Christianity. And the cult of Sol Invictus was convenient in other respects as well -respects which both modified and facilitated the spread of Christianity.
By an edict promulgated in A.D. 321, for example, Constantine ordered the law courts closed on `the venerable day of the sun', and decreed that this day be a day of rest. Christianity had hitherto held the Jewish Sabbath Saturday as sacred. Now, in accordance with Constantine's edict, it transferred its sacred day to Sunday. This not only brought it into harmony with the existing regime, but also permitted it to further dissociate itself from its Judaic origins.
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In such eclectic and ecumenical gestures, the emphasis on unity can be seen again. Faith, in short, was for Constantine a political matter; and any faith that was conducive to unity was treated with forbearance. While Constantine was not, therefore, the "good Christian' that later tradition depicts, he consolidated, in the name of unity and uniformity, the status of Christian orthodoxy. In A.D. 325, for example, he convened the Council of Nicea. At this council the dating of Easter was established. Rules were framed which defined the, authority of bishops, thereby paving the way for a concentration of power in ecclesiastical hands. Most important of all, the Council of Nicea decided, by vote, that Jesus was a god, not a mortal prophet. Again, however, it must be emphasised that Constantine's paramount consideration was not piety but unity and expediency. As a god Jesus could be associated conveniently with Sol Invictus. As a mortal prophet he would have been more difficult to accommodate.
In short, Christian orthodoxy lent itself to a politically desirable fusion with the official state religion; and in so far as it did so Constantine conferred his support upon Christian orthodoxy. Thus, a year after the Council of Nicea, he sanctioned the confiscation and destruction of all works that challenged orthodox teachings works by pagan authors that referred to Jesus, as well as works by "heretical' Christians. He also arranged for a fixed income to be allocated to the Church and installed the bishop of Rome in the Lateran Palaces Then, in A.D. 331, he commissioned and financed new copies of the Bible.
This constituted one of the single most decisive factors in the entire history of Christianity, and provided Christian orthodoxy the "adherents of the message' with an unparalleled opportunity. In A.D. 303, a quarter of a century before, the pagan Emperor Diocletian had undertaken to destroy all Christian writings that could be found. As a result Christian documents especially in Rome all but vanished. When Constantine, commissioned new versions of these documents, it enabled the custodians of orthodoxy to revise, edit and re-write their material as they saw fit, in accordance with their tenets. It was at this point that most of the crucial alterations in the New Testament were probably made, and Jesus assumed the unique status he has enjoyed ever since. The importance of Constantine's commission must not be underestimated. Of the five thousand extant early manuscript versions of the New Testament, not one pre-dates the fourth century." The New Testament, as it exists today, is essentially a product of fourth-century editors and writers custodians of orthodoxy, "adherents of the message', with vested interests to protect.
[Source: Baigent, Leigh, Lincoln - Holy Blood, Holy Grail]