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I think that Constantine the Great was just the last drop in the already filled vase. Changing his decision would have little or no effect on the spreading of Christianity in the Roman Empire.

First of all, Roman religion was intrinsically open to external influences: there are several accounts of foreign gods which were, more or less, accepted or tolerated in the Roman Pantheon or religious behavior. When Christianity started to spread, it was just one among the others.

Moreover, around the 3rd century A.D. the old corpus of the Roman religion, with its values and traditions, was already showing crisis signs: internecine wars were common in the empire, barbarians were already pressing the empire's borders and the economic crisis was hitting the empire.

No wonder that there was lack of trust in the old gods, which were showing more and more how they were no longer able to ensure prosperity and safety to the empire worshiping them.

If you really want to prevent or reduce the spread of Christianity in your world, I think you have to work on those factors I have listed before:

• openness of the religion: make the Roman religion much more closed and afraid of accepting foreign cults


• internecine wars: reduce them, make the central government strong and respected


• barbarians at the border: again, make the central government strong, so that the army is efficient and used for protecting the borders, not for supporting generals fighting for their seat in Rome


• economic crisis: I have no knowledge here to hint at what could be done, if I had I was working in ECB right now.

All in all, I think you are more or less trying to avoid that fruit flies gather around a ripe peach.

If you don’t want Jesus to just never have existed, then I’d suggest that the minimal change would be for Saint Paul to not have his vision on the road to Damascus, which would likely keep Christianity as a small Jewish sect.

This is a deeply controversial topic by virtue of the fact that it touches on many belief systems to a point where the known history is hotly debated where it doesn't necessarily agree with established dogma, especially Catholicism. The official Catholic position is that St Peter formed the first united (or catholic) Christian chruch, and that he was its first pope.

Contemporary history indicates that the first leader of any church that we would recognise as Christian was formed by Constantine, and that Christianity after Constantine bore little to no resemblance to Christianity before him.

One could argue that this was because it was deeply fragmented due to ubiquitous persecution and therefore Constantine represented the first opportunity it had seen in centuries to form around a coherent declaration of faith and practice, but the bible that was agreed to as Canon during his time contained far fewer gnostic (mystic) texts than agnostic (non-mystic) texts (the most notable exceptions to this being the gospel of John and his book of the Revelation).

The point being, there is a historical argument that Constantine didn't spread Christianity, rather he spread a religion that he created in an attempt to unite the Roman Empire under a single faith, and just hijacked the name Christianity because it was lying around everywhere (meaning he wouldn't have to explain where a brand new religion just came from).

Let's (for the sake of this essay) assume that this is the most historically accurate interpretation of what occurred.

If that was the case, he did it because he was now in charge of a massive empire of deeply divided faiths, cultures, and systems of administration. The Romans were adept at propaganda and their secular structures of government were already compelling, so converting people across the empire to a Roman culture and form of government was the easy bit. But, people get very particular about their gods and just telling people that the Roman pantheon of gods were 'the real gods' was never going to work, and Constantine knew that. He needed something new, in which all people could see aspects of their original faith so that he could sell it as a refinement; a new Roman way of seeing religion, if you will.

At this point in time, the only truly ubiquitous religion in the Roman Empire was Christianity, but it served as an underground pariah to all other faiths. This, at the very least, meant all knew of the name of Christianity, even if they didn't know their dogma. At this point in time, they couldn't have anyway because the religion was so decentralised, and worked from many disparate sacred texts.

But Constantine doesn't want the dogma; he has his own ideas for that. He just needs the name. Most Christians don't mind this so much. Their god and their messiah get an upgrade, they're not hiding from people wanting to turn them into catfood (feeding them to lions) and they're sick of fighting among themselves as to what it means to be Christian anyway.

Now for the real trick; introducing all the other elements that allow for people to see their own original faith.

Ironically, the easiest is polytheism. Instead of a God of this, God of that, you now have a Saint of this, Saint of that. Move the day of Christ's birth to the date of the Winter Solstice (or the Roman Saturnalia) because most other faiths consider that date significant, change the day of the Sabbath from the seventh day of the week (Saturday) to the first (Sunday) because it fits in better with other religions that dedicate the first day of the week to their other Gods...

The list goes on.

But why would he do this? For the same reason that the British Empire struggled in India; different religions introduce different holy days, religious practices, clean stuff, unclean stuff... You just can't get anything done unless you have everyone singing from the same hymnbook. Constantine knew that the best way to motivate and unite an army drawn from every corner of the Empire was to have them unite under a single faith, and that's easier under a single god.

So IF (and it's a BIG if; I'm not saying for a moment any of this is fact, it's just a single interpretation of events for the purposes of providing an answer) you don't want Constantine to do this, you have to give him other options.

Here's a few;

1) Don't make the Roman Empire so successful

If the Romans aren't taking over cultures left, right and centre, there's no need to unite the people. A slower rate of expansion means a slower rate of assimilation, which means people are more likely to be of a Roman faith by the time the next group are being conquered. Slower expansion and less success means no need for Constantine to unite under a single religion not Roman. Also, a less successful Roman Empire probably doesn't make it into the Middle East in the first place; problem solved.

2) Have a ubiquitous European polytheistic faith already

If all of Europe believed what the Romans did (or something similar at least), then you have no need to create something new. Most Roman gods bore a strong resemblance to Greek gods anyway; if the Greeks had been more successful in propagating their own religion before the Romans come along, Christianity doesn't get a look in.

3) Organise Christianity early

Arguably, very early Christians were hampered by a pacifistic stance to persecution. Their God was going to give them their reward for sacrifice and boldness in spreading the word, not for striking back against those who attack them. Change that single doctrine about turning the other cheek, make Jesus somewhat more Old Testament in his approach, and Constantine can't take the name of Christianity and subvert it because he's too busy fighting them as his single biggest threat inside the territory of the Empire.

4) Make Romans (All Romans, including conquered people) xenophobic

If the Romans were so convinced of the rightness of their cause, and were adept at assimilating the conquered nations into accepting that belief right off the bat, Constantine (again) can't use Christianity the way he did. You'd have a civil war on your hands right off the bat. In point of fact, by the time Constantine gets into power, the Roman Empire is suffering from all sorts of growing pains beyond Roman technology to fix and Romans are starting to wonder about their Gods as it is, what with Barbarian invasions, economic issues, etc.

Put simply, Christianity was an idea (arguably Constantine's idea) whose time had come and it was either the second chapter of God's will on Earth after his Son, or the largest social engineering experiment ever conducted on this planet. It's entirely up to you which you believe, but this essay (hopefully) goes some way to explaining what some of the possibilities were at the time.

With or without Constantine, by the early 4th century Christianism was one of the two most popular religions in the Roman empire. Constantine adopted Christian symbols exactly because it was already popular.

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  In this alternative reality Constantine the Great never embraces the Christianity...

  
  
  

  ... and loses the civil war.

  
  
  

  The situation was that the Roman empire was, officially and in principle, a tetrarchy, with Constantine as (irregular and contested) Augustus of the West, Maxentius as Augustus of Italy, and Licinius ruling the rich and stable East.

  
  
  

  By 308 CE the political situation decayed into a six-sided civil war; Constantine had the allegiance of the legions of Britannia, Gallia and Hispania, relatively poor provinces, whereas his principal opponent, Maxentius, controlled Italy.

  
  
  

  Constantine needed a simple and clear differentiator, so that people could easily understand what Constantine stood for; in a stroke of genius he adopted the banner of the monogram ☧ (Chi Rho, for Christos), thus signalling his position as emperor of the people fighting against the political establishment. He won the war under the Christian banner, and that was that.

  
  
  

  (While the other competitors used standard traditional political imagery, Constantine adopted a symbol of the religion practiced in various ways by about two thirds of the population, especially the lower classes. By placing the symbol on his banners he implicitly made a promise to elevate the religion of the masses to equal status with the established state religions.)

  
  
  

  

  
  
  

  In hoc signo vinces (under this symbol you shall win); public domain image by Dylan Lake representing the character U+2627. Available on Wikimedia.

  
  
  

  After the civil war was over, Constantine (now the uncontested Augustus of the West) and Licinius (Augustus of the East) signed together the Edict of Milan which recognized Christianism as an accepted religion in the empire.

  
  


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  ....the Roman Empire remains polytheistic.

  
  
  

  By the early 4th century AD the traditional Roman and Greek religions were on life support; basically nobody practiced them any longer except as required by state ceremonies. They did remain in use as a source of literary and artistic tropes and subjects; but this is not life. Consider that in 2018 AD, Hollywood made a commercially successful movie where one of the main characters is a strong supernatural person named Thor, who wields a magical war hammer, claims to come from Asgard, and to be the son of Odin: this doesn't mean that the old Germanic religion is alive.

  
  
  

  Among the urban lower classes and in the army the most popular religions were Christianism (of many various sorts) and Mithraism. Polytheism remained strong only in the countryside; consider that the English word "pagan" comes from Latin paganus, which properly means villager.

  
  


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  So I would like to know what realistic change(s) could have prevented the spread of Christianity in the Roman Empire?

  
  
  

  Don't focus on Constantine; it is way too late. By the early 4th century the only question is whether the empire will be Christian, Mithraic, or maybe both. The traditional polytheistic religions of Rome, Greece, Gaul, Britain and so on were effectively dead by that time.

  
  
  

  As for the question as asked, what changes to make in order to prevent the spread of Christianism, the answer is simple and well-known. Kill Paul, or at least give him a strong incentive to forego transforming a marginal Hebrew sect into an attractive universal religion. Without Paul, Christians are ordinary Hebrews, one of the many sects into which Judaism splintered after the destruction of the Second Temple.

  
  

Starting with Constantine might be way late. You’ve got to nip it in the bud, which would be Paul.

That 13 books of the New Testament were attributed to him should show how much influence he had in early Christianity. Before him, it was just another apocalyptic Jewish sect. Paul not only worked to include gentiles (non-Jews) in the new faith, vastly expanding the population of possible converts outside Judea, but also personally established churches across the Empire.

Removing Paul is the single most destructive action you could take. And it wouldn’t even be that far a stretch. He was a Pharisee who had never met Jesus and actively persecuted Christians before he converted.

Another tact you might consider would be to avoid the destruction of the Second Temple in 70CE. Foretold by Jesus, the destruction gave creedence to Christianity and weakened traditional Judaism, which had centered around sacrifice in the Temple. Interestingly, the emperor Julian (the Apostate, r. 361-363), Constantine’s nephew and staunch polytheist, attempted to rebuild the Temple to undermine Christianity’s growing influence.

A further area of interest is that while Christianity - monotheist in the land of polytheism - was an illegal religion in Rome, Judaism itself was a legal cult. This is because their sacrifices in the Temple were considered sacrifices to Jupiter. Rome was fine with whatever faith you had if only you offered a public sacrifice to keep the gods off everyone’s backs.

Christianity was banned because they refused to sacrifice to the state deities. In many cases, the punishment for being a Christian came only when they refused to offer public sacrifice. That led to martyrdoms and people do love a good martyr. A loosening of restrictions might have contribute to Christianity becomeing just ‘one of the weird cults’ in the empire, not the resident neighbor-loving bad-boys who got so popular for sticking it to the man.

The Assyrians went full on genocide and wiped out the Jewish people.

No more Judaism let alone Christianity.