Another way to look at this is when Christ died on the cross, there was a concerted effort to purge the Roman Empire of Christianity. It was the religion of slaves and was causing problems in terms of the ruling class. Picture the Roman goon squads, supported by the atheists of that day, helping to track and gather those pesky Christians for death. The only way to keep their traditions alive, under holocaust conditions, was to memorize and teach. Few people had education for reading and writing, so information was passed word of mouth. This was the religion of slaves, not land owners.
By the 4th century AD, Constantine, who was the emperor of Rome makes Christianity the official religion. This was due to the courage of the Christian in battle as subjects of Rome. They were Rome's best armies, fearless in battle; no fear of death due to Jesus. The resentment of the modern atheists appears to begin about this time, because the atheist predators became the prey; payback time.
Why didn't Rome choose atheist liberalism? It had, but this was the source of the internal decline of Rome; perversion and corruption. Roman needed men and women not unnatural human hybrids. Rome survives and gets another chance through the church. The Holy Roman Empire spawns a wide range of cultures; baby bells, who would have future world wide impact and influence; England, France, Spain, Germany, Russia, Italy, Portugal, etc. The proof of the strength of Christianity was in its ability to have its hands on the pulse of history.
Actually, but not surprisingly, you are wrong on many counts. Christians were persecuted sporadically under Roman rule and for various reasons. But Christians were not just slaves, some were wealthy and some were very powerful folks. That brings us to your next error.
Constantine didn't make Christianity Rome's official religion. Constantine certainly organized the early Christian Church and funded the early Christian Church. But he didn't make it the official Roman religion. He legalized it.
"Anti-Christian policies directed at the early church had occurred sporadically and in localised areas since its beginning. The first persecution of Christians organised by the Roman government took place under the emperor
Nero in 64 AD after the
Great Fire of Rome;with the passage in 313 AD of the
Edict of Milan, anti-Christian policies directed against Christians by the Roman government ceased.
[1] The total number of Christians who lost their lives because of these persecutions is unknown, although early church historian
Eusebius, whose works are the only source for many of these events, speaks of "great multitudes" having perished, he is thought by many scholars today to have exaggerated their numbers.
[1][2]:217–233 Although
provincial governors in the
Roman Empire had a great deal of personal discretion and power to do what they felt was needed in their jurisdiction, and there were local and sporadic incidents of persecution and mob violence against Christians, for most of the first three hundred years of Christian history Christians were able to live in peace, practice professions, and rise to positions of responsibility. Only for approximately ten out of the first three hundred years of the church's history were Christians executed due to orders from a Roman emperor.
[2]:129"
https://en.wikipedia.org/wiki/Anti-Christian_policies_in_the_Roman_Empire
Point in fact, Constantine was a high priest of the Sol Invictus cult and was so until the day he died. Constantine was a warrior prince who inherited a fractured Roman empire. He used military force and the Christian religion to unify the empire. In part, because Christianity was quite popular especially among the Roman intelligentsia. Constantine's mother was a Christian. However, Christians were often fighting among themselves and that could destabilize the empire. There were many competing Christian factions at the time. Constantine unified those factions and in doing so strengthened his hold over the empire.
What is known about Constantine is that he was a very ambitious man. He wanted power and he was a brutal emperor. He murdered his own family - not a very Christian act. Constantine had a profound impact on Christianity of that there is little doubt. He in effect injected many of his pagan traditions into the Christian religion (e.g. Christmas, the Sunday Sabbath, etc.).