When did the Romans become Christians?
In the book of Acts in chapter 28 v 11-31 it indicates that there were Christians present in Rome before Paul arrived there in AD 61 or 62. He was able to preach freely for two years in Rome and so a few more people would have been added to the number of believers in this time.
Much has been made of Constantine and his seeming conversion to Christianity. Although he certainly favored the Christians and made Christianity the state religion and ended many years of bloody persecution, much of what he did was not conducive to true Christianity. For example, monetary rewards offered by him probably encouraged many to 'fake it' to get the money. The emperor offered a white garment and twenty pieces of silver to every new convert from among the poorer classes.
Christianity had spread rapidly before the time of Constantine, despite the state-sponsored persecution. As many people made undoubtedly false conversions under Constantine, there were obviously many who were still not Christians at this time. No doubt there were many afterwards as well.
Another Slant In the 313 CE Edict of Milan, Constantine I and co-emperor Licinius removed all remaining penalties associated with Christianity. This did not outlaw the existing state religion or any other religion or cult, or make Christianity the state religion - it was theoretically a neutral religious policy. However it freed Christians to take up public office to the highest level, and Constantine sponsored the Churches in exchange for their support. Official adoption of Catholic (Universal) Christianity was in 380 CE by Emperors Theodosius, Gratian and Valentinian II to resolve the schisms within the Church, which were countering Constantine's aim of the religion being a unifying factor in the empire: It is our desire that all the various nations which are subject to our Clemency and Moderation, should continue to the profession of that religion which was delivered to the Romans by the divine Apostle Peter, as it has been preserved by faithful tradition and which is now professed by the Pontiff Damasus and by Peter, Bishop of Alexandria ... Christainity was a mystery religion, along with the cults of Mithras, Isis and many others. Early mystery religions had cult centres, and intending initiates had to have the time and money to travel to them and pay the fees. When cults became missionary, they went to the customers, and so expanded greatly. Pompey's soldiers brought back Mithraism from Asia Minor (the cult centre was Tarsus, where Paul was raised) in 67 BCE, and it was spread throughout the empire by the army, their underground meeting places being found from Hadrian's Wall to Hungary and Asia Minor. Constantine was an initiate of Mithraism, but selected Christianity as a means of control because it had an heirarchy of bishops and was therefore easy to direct, unlike other cults. The spread of Christianity was as rapid as the other cults, because in an environment where Hades was a piece of blotting paper, like the Jewish Sheol, the promise of an afterlife was attractive. And uniquely even slaves could partcipate. The cult of Christ came to you via missionaries, and offered anyone, high or low, afterlife free of charge. Whether it preceded Paul to Rome, or whether Paul even went to Rome we have no direct evidence. The cult certainly spread through the Greek cities of Asia Minor to Greece and right around the Mediterranean. Unfortunately it met in private houses - apparently secretively - and conducted ceremonies like eating human flesh and drinking blood. Things like this gave the appearance of political revolutionaries, who habitually bound themselves with horrible sacrileges to avoid informing. And as the Romans were always seriously concerned about revolution, the cult was from time to time banned and severe punishments handed out when suspicions were aroused in various locations ( graphic illustration of this is found in the letters betwen Pliny, governor Bythinia, and Emperor Trajan). Nevertheless, the cult spread through the empire, along with the others including Judaism. With a better understanding, the repressions of Christianity became sporadic and linked to specific local problems, and it began to rival Mithraism and its allied Isis cult. As said before, the official merging of the Imperial cult of Sol Invictis (from which the halo) with Christianity by Constantine the Mithraist was a pragmatic act designed as a stabilising force binding the empire together with a uniform religion.