At first, Christianity wasn't super successful. There were many other mystery religion in circulation at the time. In the Roman Empire, there was something called "Roman syncretism", various religions were tolerated as long as they didn't disrupt the social order.

Christianity was persecuted at times, but there wasn't a sustained campaign of annihilation over the centuries it took before Christianity became the Roman state religion.

Why Christianity succeeded were other religions failed is an interesting question. Mithraism was another mystery religion that was popular around at the same time, but Mithras was a war god at a time when Rome was consistently getting its butt kicked on the battlefield. By contrast, Christianity was a apocalyptic religion at a time when it seemed to Romans that the world really was ending. So the message of Christianity resonated more with the peoples of the Roman empire I think. Plus it appealed to the broad masses for whom other religions like the Greco-Roman pantheon offered little (slaves and women were more likely to convert in the early days).

Emperor Constantine converted to Christianity and gave it favored status in the empire. Once the Christians had become powerful enough the struck out at the other religions, sending black-robed monks to ransack pagan temples and destroy statues. Other religions were ultimately banned, but as others have pointed out a lot of beliefs were incorporated into Christianity (e.g. Christmas was originally Saturnalia) making the transition smoother.

Christianity became the official religion of Rome after Constantine saw the Chi Rho in a vision before the Battle of Milvian Bridge.

Before that, it was a widely persecuted cult, and wasn’t even considered a religion by contemporary Romans. The thinking at that time was that religions belonged to different ethnic groups/nations; since Christianity was open to all individuals, it was seen as a cult, like that of Mithra.

Christianity was persecuted because 1. It’s always nice to have a scapegoat and 2. Christians wouldn’t offer burnt offerings to the health of the emperor. In contrast, the Jews in the empire would do so on behalf of hashem, but Christians were not permitted burnt offerings by their beliefs. Not burning an offering for the emperor was seen as treason. Roman religion was much more intertwined into its politics than any modern system.

I am surprised on the amount of false and half true responses to your question by what are clearly amateur historians or history enthusiasts. If I were you, I would try posting this in r/AskHistorians. They have more stringent requirements for comments that make sure the responses are detailed and cite credible sources.

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As a Christian theologian (in training), my answer is that the expansion of Christianity was guided by the Holy Spirit. But that probably wouldn't be a proper answer for the subreddit.

However, there are many little things that helped Christianity. The first is the inherent urge to evangelize. It is at the core of the religion and expressed clearly in the religious texts (like Matt. 28:18-20). Other religions in the Roman Empire didn't usually have this same urge.

Second, as is noted elsewhere on this subreddit, the Christian religion was not ethnically limited. While this turned some people off, it also allowed the religion to not be limited in who they would accept among the ranks.

Third, perhaps most importantly, was Christianity's mercy within a hostile culture. Children were often left to die of exposure, especially women. Christians would often come and rescue these babies, raising them as their own. Contrary to what people believe today, Christianity was also comparatively very egalitarian both among the classes and among the sexes. The early church was largely composed of slaves, lower class, and women. This appeal to a massive audience within the Roman Empire helped the spread.

It's worth noting that most contemporary religions, to include Christianity, promise 'heaven' after death. Almost all really old religions were based on a need...you pray to the gods for rain, for sunshine, etc. People would 'fall away' from those gods during times of famine, drought, etc. But by post-poning personal fulfillment until after death, Christianity assured itself of things to come in the after life.

Correct me if I'm mistaken, but from what I understand about Christianity is that they allowed people to continue their own traditions and celebrations but just with a new twist. So it wasn't like people had to abandon their previous belief system. That's why Christians today celebrate the pagan holidays of Christmas and Easter.

Christianity as an independent religion took several hundred years to evolve from a Messianic cult of Judaism. Its structure and dynamics of conversion were significantly different after it became the preferred and then official religion of the Roman Empire.

Outside of the Empire conversion became intertwined with conquest: a major shift from it being a communal religion of the dispossessed as mentioned in other comments

Secondly most of the comments relate to the experience within the core of the empire. The situation outside of the Empire such as Ireland, never conquered by Rome, is different. What little evidence we have suggests an introduction of early Christianity (before Constantine) through farm monastic settlements that were superseded by later post Constantine Christianity based on church territories, politically allied with secular powers.

However the dynamics as to how and why the early introductions happened is speculative because of lack of any definitive evidence. Attempts have been made to associate it with the cultural character of the peoples of Ireland but these are unconvincing

Hinduism, as far as my limited understanding goes, with its background and status of Vedic literature was successful through the slow absorption of fragmented local beliefs. For instance local deities were claimed to be avatars of Shiva and thus became part of the Hindu religious family

Don’t all religions have to start with a small handful?

I mean, logically is there a religion that started with a million people?

Christianity had many competitors in its early days: the old Hellenic religion, Judaism, and the mystery cults of Sol Invictus and Mithras. There was also the Roman state religion, i.e. deification of the augusti, which one could practice in accordance with many established religions (with Christianity being a notable exception).

First, the spread of the teachings of Christ through the written word between learned people of clerical and priestly classes. Koine Greek being the intellectual lingua franca of a vast area (which was also highly urbanized and sophisticated by the standards of the time) allowed these writings to be spread very efficiently.

Christianity also had broad appeal, especially to the lower strata of society: slaves, laborers, the poor, etc. They may have been socially disadvantaged, but this group was numerically superior by a vast margin. Its central tenet and promise was that if you accepted Jesus Christ as your lord and lived by his teachings—which, for someone who is poor, humble and generally a good person, is not far out of reach—then you will be rewarded in the afterlife with eternal bliss. Life was tough for a lot of people, and the idea that they would gain entry to paradise if they persevered and kept their faith was highly attractive. Despite being God, Jesus lived as they did; he worked, he ate, he drank, he experienced pain and suffering, and he did it all for them; that's a powerful and deeply personal message when most gods you know live on top of a magic mountain and throw thunderbolts around for fun.

Perhaps most importantly, Christianity was a prosleytizing religion, unlike Judaism. The conversion of non-believers was a core tenet of the faith. Throughout the religion's entire history you have evangelists, missionaries and other holy men spreading the faith, many times risking their lives (and often losing them) to do so. That kind of hardcore belief and obstinance has a snowball effect.

By the late 4th century, when the Roman Empire adopted Christianity as its state religion, the church had imperial resources at its disposal; it spread, co-opting local traditions, gaining adherents and building upon its successes. Its deep association with the Roman state continued to thrive in the Eastern Empire long after the fall of the West. And while the Western Roman Empire might have failed, the church carried on many of its clerical, bureaucratic and administrative traditions in the new kingdoms that formed from its ashes.

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The Mormon religion popped up in the last 190 years. Scientology started in 1953. Religions starting are not something old, and how many cults are currently out there? At least 1 study indicates there are currently 2,000 to 5,000 in the US alone.

When it comes to religion, there is an easy term for how they spread, "Intolerance wins". Combine that with a healthy dose of class warfare.

It's pretty complicated to try to explain the concept because it happens in different ways depending on the region, culture, and any other factor you can imagine. A very general explanation is that if a religion doesn't tolerate other religions existing it will eventually overtake and wipe out that religion. Rome's traditional thought of Pantheon of Gods actually functioned more like multiple religions. But those religions never held serious sway over one another, worst yet they were relaxed and allowed other religions to exist alongside their own. It wasn't a situation where the followers of Jupiter intentionally wiped out the followers of the other Gods saying you can't exist. Christianity stepped into this scenario and swept up quite a lot of people, mainly the poor outcasts of society, as the polytheistic temples catered mainly to the rich. Eventually through the years Christianity came to dominate, when Constantine converted a huge chunk of the country was already Christian.

Similar situation that many are familiar with is the Vikings and how the wiping out of Asatro came because the Vikings didn't care what religion people followed, but the Christians certainly did.

Other examples from Asia come to mind Burma had an early form of Buddhism I believe that was very loose and all about partying and getting drunk, one of the kings wasn't a fan of this and on a campaign came across a very devout version of Buddhism (I believe Theravada) and converted and brought it back to Burma with him and through some law making and the situation of these Monks aren't trying to rob me and get drunk and have orgys all the time, converted large portions of the population.

Thats the gist of it.

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Dunno about Hinduism, but Christianity pretty much grew like other mystery religions in Rome, to the extent that 5-10% of the population practiced it by the time Constantine rolled around. It was def not the only mystery religion open to everyone--worship of Isis was also open to women and men, slave and free.

What made Christianity unusual was that post-Constantine, it became the sole religion of the empire, and draconian rules enforced its status. Theodosius in 380 began persecuting pagans, and emperors following him added to the persecution, which included destruction of pagan temples, objects of worship, public celebrations, and holy days. Remaining a pagan turned into social/actual suicide, as non-Christians faced loss of status, loss of property, loss of opportunity, exile, and, finally, death, by 435.

There's a period of history called the Axial Age, period of about 2500 years when virtually all "modern" religions arose in recognizable form, including Christianity, Rabbinic Judaism, Hinduism, Confusionism, Buddhism. (Depending on how you define the time period, Islam occurs either just within it, or just after.) This is also when the great Greek philosophers are writing, and many other intellectual revolutions are taking place, across the globe, seemingly in societies that have no interaction with each other.

Now, there's quite a bit of dispute about whether the "Axial Age" is even real, and if it is, what it might mean.

But the thing that stands out to me about that period of time, and the religions that arise out of it, is that empires had arisen, they became considerably larger, and cities became much bigger as agricultural surplus grew. We don't have a lot of textual evidence for pre-axial indigenous religions. But they seem like they were more concerned with appeasing and pleasing gods/ ancestors/supernatural forces. Whereas the philosophies and religions that arise from the axial age are very concerned (in comparison) with questions of interpersonal ethics, and how supernatural forces etc want us to behave towards our fellow men. It's possible that the rise of the great modern religions that we know today coincided with people having to ask, "how do I live in this city of 100,000 people and not end up with everyone murdered?" instead of "how do I live in this tribe of 1,000 people who are all at least kind-of related to me??"

It also raises an interesting question of whether those great religions of the axial age are now bleeding adherents left and right because they're simply not built to answer the question when it's another couple of orders of magnitude larger -- "how do I live in this dense urban environment of 10 million people, especially when I know that the lifestyle that makes this possible is harming the planet in irreversible ways." The religions that were dominant for the last 2,000 years don't seem to be doing a great job of addressing that -- and even the ones that are seriously trying, a lot of people don't seem to find their answers persuasive. It's possible that you're currently watching the next great shift in philosophical and religious thought, and will get to watch new belief systems arise and rapidly gain large numbers of adherents in real time.

Or maybe not! Check back in 2000 years.

When an empire controls over half of Europe and forces everyone to join a religion, that religion spreads pretty quick.

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The most common usurpation was probably from dominance. If you prayed to a god or a pantheon to protect you and your civilization was overrun, it would be a pretty tangible argument for whatever belief system your conquerors brought.

Interesting topic, I think the secret lies on how preachers do their job, and the core values they deliver

Because polytheism doesn't scale, and shamanism really doesn't scale.

I mean, I can only shed the light on Christianity as this is the only topic of religion that I am fairly familiar with and I will mostly rely on two works: How our world became Christian by Paul Veyne & Rise of Western Christendom by Peter Brown. Firstly, we need to understand something that was a fundamental difference between ancient roman religions and Christianity, ancient roman religion was manly described by the term "religionis", which mostly means observance of rites & customs. According to Veyne the Romans often had slightly "diplomatic" approach towards religion. They performed the rites and sacrifices and on this basis demanded favours from Gods. Besides that, it was also common that religion, myth and though became quite separated, which created a problem of belief. Christianity on the other hand used term "credo", which means believe, as the very basis of religion. Being a Christian demanded some actual faith in the gospels, one couldn't be a Christian without believing in the gospels. Late Roman pagan on the other was more of a neoplatonist than anything else.

That focus on belief, a story of Jesus & quick use of the ancient philosophy for apologetics created Christianity quite particular and that's why Paul Veyne (who is a disciple of Foucault & Nietzsche btw) described Christianity as the "masterpiece of religious imagination", and we must be aware that the Christianity was a hot topic of debate in the educated Roman circles, also due to works of people like Tertullian. Besides that many thing that were addressed by Christianity were already a common point of antique philosophers and in a way we can say that Christianity is a philosophical sect for the masses. However, we must also be aware that Christianity weren't eschatological destiny (it was also replace by Islam quite easily) and that its victory wasn't really sure and we can surely imagine other turn of events in the case of Julian's victory. However we must also be aware of the advantages that Christianity hold over ancient paganism.

As for early Roman period we must be aware of one thing; the rupture between late roman empire and early medieval Europe was stronger on the economic terms than on cultural terms. Barbarian tribes were already under Roman influence to some extent and Christianity was in a way also a path to the inclusion in the imperial legacy. We can also ascribe similar factors as before which gave Christianity some kind of advantage over the ancient paganism, tho.

> presumably starting with a mere few dozen disciples--managed to proliferate so rapidly across different languages and cultures (from Rome to Egypt to Iran, etc.) despite facing competition and even persecution from other, previously established religions (such as those in India and the Roman Empire)

All those assumptions are questionable. There’s very little data on how Christianity got started and what happened for the first few centuries. Maybe it started with a few hundred followers and grew at a fairly “normal” rate, just a little faster than others.

In many places, Christians weren’t much persecuted. Ancient people were generally live-and-live about religion. Certain individual emperors would get concerned and try to persecute them, but their policies didn’t necessarily take much effect on the ground. Persecution often backfires and creates inspiring martyrs, then and now.

It does appear that fairly early on, the church was unusually literate and organized, which may have helped them grow faster than others. Maybe it was a coincidence from having a relatively urban, Jewish base. Also, the “established” religions weren’t really autonomous organizations, they were outgrowths of societies and ways of life. So, if you moved from rural Iran to Antioch, or were captured and enslaved, you might lose any connection to your original religion and be ready to pick up something new.

Mental illness?

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Islam is the oldest religion starting with adam and ave. A muslim isnome who submits there will to the Almighty creator, i can prove jesus was a muslim, jesus spoke aramaic, google God in Aramaic.

They all worshipped the sun and moon anyway, so plug and trace, easy peasy. It was all about eclipses and zigzags. I'm an expert!

Sometimes religions and spiritual practices evolve through exchange whether it happens naturally or forcefully. Sometimes if you want religion to catch on, you have to package it so it is appealing. Christian holidays are established to correspond with pagan holidays or certain Christian figures reflect the loa in voodoo.

Christianity spread quickly because it was applicable for everyone at the time. Martyrs willing to die going against the Roman Empire as well as question some of the Jewish laws. It accepted everyone and anyone can become part of the religion. It really became more mainstream when Constantine, became the first Christian emperor as mentioned in other posts.

There is typically a top-down political component to cultural shifts like you're asking about where governments chose the religion for the people. Additionally, times of political chaos make people more willing to accept new belief systems. The many civil wars of Rome facilitated the adoption of Christianity before it became the state religion. Similarly the political instability at the collapse of the Han Dynasty helped Buddhism gain acceptance. A happy afterlife is especially inviting in such tough times. Otherwise, when empires or kingdoms actively patronized a different faith than they had before, they did so for political reasons. In ancient Egypt, Akhenaten promoted a small time cult over the traditional religious figures to weaken their priesthoods. Even the Protestant Reformation's success was due to the patronization of local leaders in the Holy Roman Empire. Henry VIII of England couldn't get the pope's approval for an annulment so he made a new church. I'm sure there are counter-examples of bottom to top religious changes, but prior to religious toleration, the church and the state were typically hand in hand.

Vast and prolific human stupidity; the same way every other religion started.

Politheist religions began merging their gods, having fewer and fewer with the passing of centuries or even millennia. I’m not talking about Greek or Egyptian gods, but the gods of the isolated and different tribes. With the commerce and war and syncretism of the conquerors these gods merge. Also, for economic purposes there was en emphasis on the individual contrary to the older gods which emphasized on the tribe. Here Christianity and other judeochristian religions were more skilled to give the universe and world purpose by relying in the already syncretized beliefs of other cultures. The book Homo Sapiens by Yuval Noah Harari explains it better

For those saying the Christmas and Saturnalia are related: How do you explain the birth of Jesus during a census in Bethlehem? Would there be a census and festival going on at the same time?

Monotheistic and pantheistic religions have amorepersuasive message than old-line paganism, in general. In paganism it's random chanc eo r you didn't do the right ritual, in world religions it's part of life.

Gestures to Qanon

Anything is possible.

U.S. Mormonism represents a good modern example. As explained to me by those that study this issue, the expanse of Mormonism was basically done by the women. One woman talking to another etc. I would assume this also applied to Christianity.

The Baha’i Faith got smashed with incredible persecution and genocide. In the east, it’s still dangerous to be a Baha’i. However, the structure of the Faith is to spread it in a gentle way over whatever place you can. Members are encouraged to travel internationally and do what is essentially missionary work to help show the world what Baha’is believe. In those places the stories and songs are spread at “firesides” not by keeping it all in a religious building somewhere (although there are places of worship, it’s not where or how the Faith is spread). The Baha’i Faith is one of worlds most rapidly growing religions after only developing in the last 300 years. I think what most successful religions have in common, is that they build on an established religions existing beliefs and by doing it gently. The Baha’i Faith is an Abrahamic religion, building directly on from established Abrahamic religions with the belief that all of the world’s religions actually worship the same God. Edit: it also helps that the laws of the religion are to set aside some money for the Universal House of Justice in your will. Nothing happens without money. Baha’i inheritance laws are actually very interesting.

Two components:

1. By making more generous promises for people who chose to be gullible.
2. Exclusivity rights

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1. Generous promises

For example, some pagan religions allow possibility for life in afterlife.

Christianity offers, resurrection for those who live according to their beliefs. Islam offers for certain people 72 virgins in the afterlife.

In Soviet Union, socialism has been made a de-facto religion and people were promised that their basic needs are addressed (inexpensive food, medical care, shelter), and after 20 years everyone would live the lifestyle of rich people who can chose whether to work or not

2. Exclusivity rights

If the teritory has elements of the statehood a ruler or (or ruling class) can elect that one religion is to be used in the territory exclusively. After some generations people cannot imagine their life withouth the religion that was forcefully imposed to their ancestors.

Christianity started becoming wide spread when the Roman emperor ( I can’t remember his name ) had an experience that he believed God was talking to him and he made it the official religion of the realm

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