How did Christianity become Rome's official religion?

Answer 1

Christianity was a mostly tolerated, but occasionally fiercely persecuted, sect in the Roman Empire until the conversion to Christianity of Emperor Constantine, probably in 312.

Up till that time, the Romans believed that their devotion to the traditional Greco-Roman cults making up the Greco-Roman religion was the cause of their military successes, and the reason the empire had become great.

The Romans did not officially favour one god over another, but they thought that all of the gods contributed to their success.

However, Constantine came to believe that it was specifically the Christian god who aided him in his victories, in particular the victory at the Milvian Bridge, outside of Rome, over the usurper, Maxentius. This threw the whole previous system into confusion.

It came to be that the Christian emperors transferred to God, the Christian god, the former idea that it was the traditional gods who preserved the power of Rome.

It happened that the hold of the traditional Greco-Roman cults over the people declined during the fourth century, with the senators in Rome being the most notable exception.

Over time, the idea became more acceptable to make Christianity the official religion of the state, but it actually only happened under Emperor Theodosius (379-395), who progressively constricted the old cults, leading eventually to them being proscribed (banned).

In the pre-Constantinian era, and possibly for some time after that, haruspicy (the practice of finding the will of the gods by examining the entrails of sacrificed animals) was used to determine the will of the gods in regard to matters of state, such a launching a war, or negotiating a treaty.

Under Theodosius, this practice was banned on pain of death. This was, in one sense, a continuation of the old policy that only the leaders of the state held the priesthoods, so only they could examine the entrails of the animals in this way. It was a matter of political control of the affairs of state.

Theodosius' changes met some resistance, and even strong resistance in Rome itself. This included an attempt to convince him to restore the (traditional) Altar of Victory to the Roman Senate, the nominal seat of political power. There was even an attempt to install Eugenius as western emperor. Since Eugenius had been a priest of the old cults, it could be expected that Eugenius would be favourable to the old religion. That failed when Eugenius was defeated by Theodosius, who refused to accept him as his imperial colleague in the western half of the Roman Empire.

Thus it was Emperor Theodosius I who made the traditional cults into an illicit (not officially permitted) religion in the Roman Empire, and from that time Christianity (according to the Nicene creed) became the only religion that was not disadvantaged by the state.