Christianity laid forth rules of life that were wise and effective. — Tzeentch

Just as it is "wise and effective" for lions to hunt antelopes.

Do you think that accounts for 100% of them at all times? — Tom Storm

Sure, they occasionally forget their doctrinal tenets or stray from them ... But the ideal has always been supremacy.

Persecution of Christians in the Empire before Constantine was sporadic and local. Nero's efforts were limited to the city of Rome, for example. Persecution was seldom organized or pursued throughout the Empire. I'm afraid the persecution was vastly exaggerated by Hollywood.

This belittles the point: Christian’s were brutally persecuted throughout the early church.

~60 A.D.: Nero burned them alive as a source of light in his gardens; disguised them as animals to be thrown to wild dogs; etc.

~80 A.D.: Domitian was very harsh on Christians. Most notably putting St. John the Evangelist in boiling oil and exiling St. John the Apostle (after which he wrote the Book of Revelation).

~160 A.D.: During Marcus Aurelius’ reign, albeit it not directly his fault, Christianity was widely persecuted.

Etc.

The problem is that you are belittling Christian persecution because it was not oftentimes incredibly centralized to the highest government. Christian’s widely had to meet in secret, were executed for their faith, blamed for every problem with rome, etc.

In fact, Christians were notorious for their eagerness for martyrdom. Tertullian actually boasted of this death wish. He wrote of an incident when a crowd of Christians accosted a Roman magistrate and demanded he kill them. The annoyed magistrate told them that if they wanted to die so badly they could find rope to hang themselves or throw themselves off a handy cliff, but he wouldn't accommodate them.

You make it sound like they were begging romans to kill them: that’s simply not true. Tertullian encouraged Christians to endure persecution—up to and including death—for the faith because of the horrific persecution that was going on. He wrote in the Apologeticum:

Christians are persecuted in ignorance, because they are not allowed to defend themselves - as long as they can be called 'Christians', they can be executed. Real criminals are allowed to deny their offences, defend themselves, and are tortured to get them to confess. By contrast the Christians are not allowed to demand evidence of any crimes they are condemned for, and are tortured to make them stop confessing. Christians are denied any chance to vindicate themselves, nor do the magistrates try to find any evidence of crime - the name of 'Christian' is enough.

(https://www.tertullian.org/works/apologeticum.htm)

You are twisting history into this phantom of rome where Christian’s lived normal lives most of the time but had this fetish for martyrdom; and that is simply ahistorical.

The doctrine of forgiveness of sin provides a method to avoid responsibility. Why be virtuous when you can always be absolved on request?

This would be a fair point IF asking for forgiveness was repentance. You seem to be under the completely false impression that if you simply ask of the Jesus to forgive you that you are forgiven: that’s not the historical view. In Catholicism, our salvation is caused (1) meritoriously by Christ’s sacrifice, (2) efficiently by God’s grace, and (3) instrumentally through our participation in God’s grace. This means that sola fide should be fides caritate formata: love is works and faith. We are saved by our genuine love for God and our participation in the Sacraments to elevate and maintain a state of sanctified grace.

Doing a sin with the intention of immediately repenting afterwards is itself a sin requiring repentance; and it is a mortal sin since it (1) has grave matter, (2) was intentional, and (3) had full knowledge. To be clear, this means that in your scenario here where someone avoids trying to be the best person they can be for God’s glory because “they will get saved anyways through repentance” will go to hell.

Although Kings often battled with the Church, Christianity offered philosophical support for Monarchy. After all, God rules in heaven. I'm reading "Paradise Lost" now, and Satan rebels against the autocracy of God's rule/ His rebellion is a noble one, although, like other revolutionaries, he doesn't want to change the system, merely his role in the system. It is better (he thinks) "to reign in hell than serve in heaven."

The divine right of kings mirrors God's rule in heaven. But if coercive force is a bad thing, mustn't utopia be an anarchy? Heaven and Hell suffer from the same flaw: autocratic rule.

I find the period during which the Roman Empire transitioned from a largely tolerant polytheistic society to an intolerant monotheistic society fascinating. For the most part, Rome didn't much care what gods were worshipped provided no threat was made, perceived or otherwise, against it or public order. Similarly, the devotees of pagan religions didn't insist that others worship only a particular god, nor did they much care what gods they worshipped.

For example, initiates in one pagan mystery cult were often initiates in another cult.

Jews and Christians were different, however. Their religious beliefs were exclusive and intolerant. The Romans had an odd regard for the Jews and their ancient, tribal god for a time. The Herods were clients of Rome. But when the Jews tried to throw off Roman rule (twice) the response was terrible.

Also, there was sometimes religious violence between Jews and Greeks in cities like Alexandria. The belief that one's own personal god is the only God and all others false seems to encourage repression and violence.

The Christian desire that everyone should worship Jesus and insistence that they do so and should be compelled to worship no other gods far exceeded that of the Jews, however. It eventually lead to the destruction of pagan world, though that world survived in certain ways through the Christian assimilation of certain pagan religious traditions, and sometimes even pagan gods via the cult of the saints.

I wonder how and why this enormous alteration in the ancient world took place.

All ancient civilizations exhibited religious tolerance (except the Jews). If you traveled to another region of the known world, your first task was to find the local temple and pay homage to their gods. The Romans were like everyone else in this.

Isaac Asimov said that the Jews invented religious intolerance and became the world's first victims of it in the 6th Century BCE when the Babylonians invaded and specifically attempted to destroy Judaism. Christianity inherited this preoccupation with truth. You're supposed to realize that the gods you've been worshiping aren't real. And Northern Europeans did realize that. The destruction of paganism didn't happen at the end of a Roman sword. The pagans destroyed their own culture(s). They burned it all.

It makes them feel superior — baker

What position is the person in who says about another person “it makes them feel superior?”

That doesn’t seem right. Pots and kettles scrapping for the superiority of their color.

How do you measure success to a Christ hung and bled to death on a cross in the public square? You have to account for Christ at least a bit when you ask about Christians. Christ is the message. Christ is what Christianity is. Not human history.

“feel superior” - how many times did Christ implore anyone who would follow him to serve, to never desire to be first or greatest? The night before he died he washed the dirty feet of his students.

Feel superior - that’s soft analysis of the legacy of Jesus Christ, if you ask me.

the ideal has always been supremacy — baker

For the first approximately 300 years (that’s 3 centuries) how many Christians felt superior then?

Seems like a solid foundation in humility to me. Not supremacy at all. Christ was God, and he never did anything but what his father told him to do, unto death, on a cross, at the hands of we pigs and rats. Find the superiority over others in that!

What is the “success” of Christianity, anyway?

The fact that so many people call themselves Christian? Is that the bar for success?

If it is some sort of worldly dominance, or numbers game on converts, that’s worldly, that comes and goes, that’s petty. That’s not Christian success, if you hear what the gospel preaches. That’s stuff for people who count stuff as “success”. Christ didn’t count such things.

My understanding of a Christian success would be sainthood. How many saints do you think there are? Having met many people in my life, I suspect not many. Who gets to judge the most successful religion now?

But my straight answer, talking history or psychology, Christianity is the most widespread through history and across the globe because it is the most practical (easy rules) and welcoming of all religions, calling sinners first and foremost (so every single soul is wanted). And my answer talking theology is that the success is mostly because God wants it that way. The success of Christianity is more proof of the existence of the Holy Spirit in the world, working through history, despite all of our competing earthly “success” stories.

Regarding the exaggerated accounts of persecution of Christians by the Roman Empire, you might consider reading The Myth of Persecution by Candida Moss, Professor of New Testament and Early Christianity at the University of Notre Dame.

The saint supposedly thrown into boiling oil also supposedly lept out of the cauldron unharmed, miraculously. I don't think such stories very credible. Just who wrote the Book of Revelation is a matter of some dispute.

Tertullian in Ad Scapulam, Chapter 5, wrote of all the Christians in the province of Asia presenting themselves to Gaius Arrius Antoninus at his judgment seat, and his comment to them: "O miserable men, if you wish to die, you have precipices or halters." Tertullian seemed to be arguing that Roman cruelties to Christians was their glory, and that Christians "even invite their infliction."

I think knowing that your sins will be forgiven if, sometime in the future, you really, really repent and seek forgiveness renders wrongdoing of less significance now. It's not that you intentionally do wrong because that option exists, it's that doing wrong becomes something there is less need to avoid if the fear of divine punishment is what keeps you from sinning. Ideally, one should be virtuous for the sake of being virtuous.

The saint supposedly thrown into boiling oil also supposedly lept out of the cauldron unharmed, miraculously. I don't think such stories very credible. — Ciceronianus

Do you think 1,000 years ago anyone listening to the things human beings do casually now would take such accounts credible? Communicating with people all across the world in 2 seconds? Exploring the depths of the ocean for hours even days or longer at a time? Traveling in a flying ship carrying hundreds of people across lengths that used to take months in a manner of hours? Visiting or otherwise landing on a planetary body, even one as close as the Moon?

Honestly. Know thyself.

Just as it is "wise and effective" for lions to hunt antelopes. — baker

Then you may as well classify any societal human endeavor as "lions hunting antelope".

The Christian desire that everyone should worship Jesus and insistence that they do so and should be compelled to worship no other gods far exceeded that of the Jews, however. It eventually lead to the destruction of pagan world, though that world survived in certain ways through the Christian assimilation of certain pagan religious traditions, and sometimes even pagan gods via the cult of the saints.

I wonder how and why this enormous alteration in the ancient world took place. — Ciceronianus

You raise some fascinating questions. Have you encountered any decent books that have explored this theme in a useful way?

Let me try to answer your central question:

What explains the success of Christianity?

1. High universality for its time – Christianity's ability to explain various areas.
2. High productivity – Christianity's ability, once accepted as the norm, to generate new, logically necessary, non-trivial consequences that could not be derived from previous experience.

One I found interesting is The Final Pagan Generation: Rome's Unexpected Path to Christianity by Edward Watts. One I'm beginning to read that looks promising is Christendom: The Triumph of a Religion, AD 300-1300 by Peter Heather.

The Christian desire that everyone should worship Jesus and insistence that they do so and should be compelled to worship no other gods far exceeded that of the Jews, however. It eventually lead to the destruction of pagan world, though that world survived in certain ways through the Christian assimilation of certain pagan religious traditions, and sometimes even pagan gods via the cult of the saints. — Ciceronianus

Are “compulsion to worship no other gods” and “assimilation of certain pagan traditions” a bit at odds? There was the Inquisition, and its coercion, but that was not close to assimilation. (And that wasn’t Christlike or Christian, so should account for any “success” of Christianity.)

High universality for its time – Christianity's ability to explain various areas. — Astorre

That makes sense. And it explains Christianity’s ability to assimilate new people’s traditions. Christians came to a new culture, sought what was universally good in it and in its people, and found what was good about that culture’s relationship to the divine, and thereby found the universal spirit of their one God already working in that new culture. Assimilation was growth for everyone.

High productivity – Christianity's ability, once accepted as the norm, to generate new, logically necessary, non-trivial consequences that could not be derived from previous experience. — Astorre

You are talking about high productivity of ideas. I agree, and would link that eventually to the production of universities. And notice the word “universal” in the university. (And the word Catholic means universal as well.)

I would also add charity in deeds is very productive and convincing of would be converts. Seeing a new priest share his only loaf of bread with family, or teaching the poor to read - that draws people together. And led to the eventual production of hospitals.

People are quick to equate religion with so many ascetic rules and with earthly-looking power structures. And they equate its spread with earthly tactics of spreading earthly ideologies, including coercion and psychological tricks. But Christianity was always different as it requires freedom to achieve its ends. The core Christian message is that God is trying to bring us to know and love him. There is no such thing as knowing someone or loving someone without their free, honest willingness. This in itself is more universally appealing.

Christianity democratized human value, not to each other, but to a God who loves each one.

In my view, it is easy to see why Christianity spread so far and wide.

People are quick to equate religion with so many ascetic rules and with earthly-looking power structures. And they equate its spread with earthly tactics of spreading earthly ideologies, including coercion and psychological tricks. But Christianity was always different as it requires freedom to achieve its ends. The core Christian message is that God is trying to bring us to know and love him. There is no such thing as knowing someone or loving someone without their free, honest willingness. This in itself is more universally appealing. — Fire Ologist

I agree with your assertion. Moreover, I'd like to point out that the question itself is already posed within the paradigm of "why did this ideology take off," rather than, for example, "is Christianity a doctrine of love?"

the question itself is already posed within the paradigm of "why did this ideology take off," rather than, for example, "is Christianity a doctrine of love?" — Astorre

Fair. I just think that without a sound, basic understanding of what Christianity is, one won’t be looking in the right places for how and why it succeeds. So I’m volunteering my understanding of what Christianity is.

It’s like wondering why the core tenants of the US constitution took off and proliferated in various forms in so many newly formed nations - if one asks why, but sees the US itself as only a colonialist, racist, freedom crushing, economically enslaving, exploitative land of uneducated cult members, then you probably won’t understand why it’s constitution became so appealing. One needs to look at the lives of the people in the US that flourished to explain why the US flourished, and why its constitutional inventions allowed for that flourishing. One needs to honestly categorize the poor US citizen, or even the US prisoner, and their station in relation to the rest of the world’s citizens and the rest of history to judge the success of the US. One wouldn’t be anything more than astonished by the success of the US if it’s constitution was merely a new mask for tyranny and crowd control.

Why does Christianity appeal to anyone? That may be enough of the answer for why it was so successful. And the answer to why it is appealing has to include some information about what it is (at least what it is to that person) (and once you dig into what Christianity is, you need to at least ask “who is Christ” and “what is His message”). And my suggestion for what Christianity is to most insiders has to do with living lives of love, charity, service, and seeking knowledge. These qualities draw non-Christians to Christianity without any effort of the Christian to convert anyone. These qualities build stronger individuals and communities. So it’s inherent appeal spread itself, and it survived/flourished by design of what it is.