Constantine the Great was the first Roman Emperor who was a Christian; with the exception of perhaps a single of his successors, he would be the first Christian leader in a long line of eventual leaders.
Prior to Constantine’s rise to power Christians were a persecuted sect in the Roman Empire. Though the Roman Empire did not care if its citizens identified as a religious force, it did care, in its pagan days, about ritual. What this meant was that people were free to worship—or abstain from worship—who they wanted as long as they did, at least on paper, uphold and ritually worship the Roman gods.
Why was this Roman policy? It can be chalked up to an insurance policy.
Roman religion, heavily focused on ritual, saw deference to the Gods as vital. For example, if one properly performed the rituals of praise to the Roman gods, then instead of pestilence and disaster, the gods would grant the performer bountiful harvests and personal fortune. So, if we take this and multiple it on a large scale, Roman policy forcing everyone to ritually worship the gods, was a kind of pseudo-economic policy aimed at curating favor with the divine.
An interesting set of historical minutia is that though both Christians and Jews were equally hostile to ritually worshipping the Roman gods, the Romans were far harder on the Christians than the Jews, despite there being several large-scale Jewish revolts. Why? Because Romans had a profound respect for all things old; whereas Judaism was very old even during the times of the Romans, and this deserving of respect, Christianity, on the other hand, was new—a novelty—and so it lacked the ability to protect itself from Roman persecution.
During this time in Roman history, Christianity is an urban religion. Because peasantry are so inaccessible to missionaries, it is difficult for preachers to convert the rural dweller. Likewise, because early Christianity would only have an appeal to those who are, in some manner, marginalized from Roman society and culture, the early converts to Christianity would only in the rarest of circumstance be from the well-off sector of society.
Women, for instance, are disproportionality represented in early Christian sects; slaves and freed slaves; immigrants, wolvers and wanderers… are among the social groups which respond well to the Christian message. Early Christianity was the socialization of the underclass; no one needed an advanced education in order to understand its tenants and premise (Roman intellectuals, in fact, would ‘turn up their noses’ at its intellectual core when they saw that it was written in this low-brow Greek). Social distinctions do not matter in early Christianity and so it has a radical edge in its content when theologically it espouses that the soul of an emperor is the same as that of a slave.
Early Christianity up-ended much of Roman society. For example, Roman society saw love of honor as a social benefit: if you pursue honor, if you emblazon your built baths and aqueducts in big, bold letters honoring your contribution to the social sphere, then it would ultimately benefit society as a whole. Christians, however, saw love of honor as pride and so as a sin. Likewise, in another way that Christianity upended Roman culture, was its view of poverty; Roman culture ascribed no virtues to being poor. Being poor simply meant being poor. Early Christian dogma, however, saw poor people as spirituality able to enter the kingdom of God in the afterlife, and so special value was projected onto poverty. What Christian theology promoted, in other words, was a sense of community and togetherness at a time where many Roman social groups on the fringe of acceptability were struggling to find their place in a social vacuum.
Emperor Taejon put forward a policy to dampen the bloodletting of Christians. Before his time, it was official policy to hunt down and execute Christian cultists. However, after his rise to power and subsequent passage of laws which refused the ability of Roman officials to persecute Christian cultists, what transpired was that Christians could only be executed if they were known without a doubt to be Christian and had publically announced their status as Christian. Indeed, from 260 to 302 there is no Roman persecution of Christianity and Christians greatest threat comes from the pagan majority whom would occasionally launch pogroms against the Christian community should crops fail or other such misfortune befall their town.
By 305, however, persecution of Christians had returned full-throttle. Provincial governors were encouraged to kill a certain quota of Christians while people were promised tax breaks for their assistance in the killings. By the time of Constantine, his conversion to Christianity was an unlikely event; convinced that it was his painting of a cross on his soldiers’ shields which led him to military victory, he becomes convinced of Christianity’s superiority and in 312 issues an edict of forgiving regarding Christianity. Though his Western imperial counter-part would forego his edict, it would become official policy once he conquered the whole Empire.