There are more than obvious political reasons why Constantine (272-337) would have turned to favoring the Christians, shortly after his predecessor Diocletian had still gorily persecuted them for the last time (303 CE).
The waves of persecutions of Christians in the Roman Empire had been very different in character. Under Nero in the 60ies CE, there was a Jewish sub-group in the city of Rome that displayed too little horror about the great city fire, because these people were acquainted with a positive interpretation of such scenery by their apocalyptic traditions, thus making themselves unpopular and suspicious almost without any further implications.
After 70 CE, increasingly there was kind of a tacit agreement between „the two new and future forms of Judaism”, saying that Rabbinical Judaism would claim to connect breachlessly to older Judaism (which isn’t totally true), while Christianity accepted to be something completely new (which isn’t totally true either). Jews had been an allowed religion in the Roman Empire before, but as religion in antiquity always was a deeply political issue, of course the Romans were not willing to split that license. Consequently, from about 90 CE onwards, there were persecutions of Christianity as an illicit religion. But these persecutions were sporadic. Trajan told governor Pliny in a famous letter: Don’t listen to anonymous accusations, put the non-denying ones among the accused to a test concerning their loyalty towards the Roman state by having them bring some little pagan sacrifice, and if they refuse to, then have them executed for their resistance against the state.
Under Marcus Aurelius in the 160ies and 170ies CE, it turned out to be not enough to persecute the illicit religion on a mere occasional basis, as the Christians now had more and more intellectuals showing up publicly and harshly attacking the allowed Jews. From that time on, it was the ideological conflict that started to be perceived more strongly, focusing especially on the Christians’ typical „un-Roman” tendencies of privatizing, inclusivity across all borders of class and gender, and exclusivity towards other religions. But as times soon turned more and more disastrous due to pandemic („Antonine Plague”) as well as intensifying border defense warfare at the Danube marking the earliest beginnings of the Migrations of Peoples, systematic altercation with Christians was cut short and turned into punctiform outbreaks of scapegoat-searching popular anger.
The establishment of the church progressed nevertheless. In the early 250ies CE emperor Decius launched the first state-run, empire-wide persecution of Christians in order to get rid of that „new“ religion before it would finally be to late to prevail against its dynamic development. Already Decius’ motive was the unifying ideological strengthening of Roman society in order to overcome the empire’s deep crisis during that century.
The Roman Empire had always very much been based on religion. After the deep crisis of the third century CE, Diocletian again wanted to renovate the empire on the basis of a cultural ultra-conservatism. But meanwhile the consolidation of the church had gone on for another half of a century. Since 360 CE, there had been the „Little Peace of the Church“, and Christianity had been flourishing in the Empire. In Nicomedia, a huge church was just under construction on a hill overlooking Diocletian’s palace when the emperor ordered his surprise persecution to unleash. This was no longer a move against some clandestine group. And it was basically already a fairly desperate, unrealistic and bizarre attempt in 303 CE. Diocletian pursued an express concept of pagan restoration, and the Christians could not be but opposed to that idea. By persecuting the Christians, Diocletian himself tragically tore his own political Opus Magnum apart instead of furthering it. After the persecution, the world had seen that this was no possible solution, as the church since long was too strong already to get rid of it that way. Which means, „Constantine’s“ church did by no means „suddenly emerge out of nowhere“.
The Roman state was actually pretty weak in terms of structure, and not just temporarily so, but by principle, by virtue of its concept. It was largely limited to military affairs and collecting taxes. Whereas the church already since the middle second century CE had begun to develop a very strong and elaborate internal structure, serving needs of authority, legitimation and control, information, communication and education, administration, joint finance and charity. Only a very stupid politician could have missed to see this institution as a marvelous potential tool of power.
Was Constantine’s favor towards the church nothing but a perfectly logical, almost compelling political move? Was he himself a lifelong pagan?
In order to counter the view that Constantine was a serious Christian, what has been referred to is that he didn’t get baptized earlier than on his deathbed, and that he ruthlessly murdered some of his family members. But these clues are no clues.
Family murder was late antiquity’s common „reason of state“, and many Christian rulers of that time still did it even after their baptism – see, for example, Clovis (c. 466-513).
Not only was baptism on the deathbed convenient to such rulers, as it safely forgave all sins, which secured them a pole position on the way to heaven, but also there was the doctrine in Early Church that sins committed after baptism could not be forgiven anymore. Therefore, it was quite common during that epoch, not only for rulers, to receive baptism late in life. In addition to that consideration, there was the further one that severe worldly penances would have been imposed on the sinner who was a member of the church – which many deemed to be unfitting with the honor of a princeps.
The emperor whose predominant goal was to reunite and re-stabilize the empire clearly would have been eagerly careful to do so by being the emperor of all members of mega-diverse imperial-Roman society. In his particular historical situation, Constantine could serve that goal best by being an emperor who was openly in favor of the Christians, but not (yet) a formal member of the church. At a private dinner with bishops around the time of the Council of Nicaea, the emperor famously called himself the „bishop of those outside the church“ (Eusebius of Caesarea, Vita Constantini 4,24), which may be regarded a proof to the aforesaid assumption.
Consequently, we simply have no hint that Constantine was not a serious Christian just because he was a serious politician.