Greco-Roman Polytheism had a lot less in common with modern monotheism than is sometimes thought. Religion had several purposes: to appease the gods and keep them on your side with sacrifice, a ritualised element of civic and everyday life, and to afford regular feasts where groups could actually eat (sacrificed) meat and get pissed.
Whatever gods you believed in, you didn't object to the worship of others: life was precarious enough without pissing off strange gods too. If people refused to take part in your rituals, like a weird little cult starting doing in the second century or so, you could get pretty annoyed though: if a crop failure or an earthquake followed, it might be best to massacre them to be on the safe side.
Believing in anything particular in the religious texts wasn't important: the stories were cheerfully inconsistent anyway. Taking part in the sacrifices and rituals was the important thing. Zeus was hardly likely to care what you believed as long as he got his share of the divine essence of beef.
The gods and texts were not sources of moral instruction: the gods were mostly interested in shagging, fighting and sacrifices. The people who worried about morals and ethics and how you should live your life were philosophers, who wandered around making speeches and criticizing the extravagant: if they were any good they were supposed to set an example themselves. Philosophers thought that the pursuit of physical pleasure actually made people greedy and miserable in the long run, and urged restraint, even celibacy sometimes
Neither philosophers or priests had much to say about life after death. A few great heroes were supposed to have been favoured into an eternal reward on an isle of the blessed, but no-one really knew what happened to anyone else.
One of the Semitic peoples of the Middle East, they moved around a bit more than most as various rulers shuffled them around their empires. They started out as conventional polytheists, but the priests of their Zeus-equivalent Jehovah promoted him very well: he moved from chief god to the only god who should be worshipped to the only God who actually existed.
Later on they got conquered by the Hellenistic Seleucid dynasty. They didn't get as Hellenized as everyone else: the priests of their single God resisted rivals, and their ritual purity laws kept them from mingling as freely. Even so, Greek ideas came across, especially since the obscure Hebrew language of their texts fell out of use in favour of the more widely-used languages Aramaic and Greek: many Jews only knew them from the Greek translations.
Jews like Philo and Josephus found things in common between Jewish thought and Greek philosophy, which had also come up with ideas about a universal, benevolent, creator-god. Now that Jehovah was a solo God with no-one to fuck or fight, he had turned out to be a source of morality.
Early and Middle Christianity
For a while a few people in the Greco-Roman world had been interested in the religion of the Jews: it was intriguingly exotic and there were a lot of them scattered around as a result of various political calamities. It didn't take off though: the purity laws were dauntingly strict and for men even involved having a bit of your penis chopped off: this was a lot more likely to happen if you were a baby male rather than an adult able to resist.
Eventually Romanized Jew named Paul, a follower of a deceased Jewish revivalist preacher, hit on some radical new ideas. The teachings of his guru superseded the old law so much that you could eat pork and retain the whole of your penis. The humiliating execution and death of the guru, formerly an embarrassment, turned out to be a symbolic sacrifice like that of the Jews' Passover lamb, which absolved you of your sins in the same way, without the inconvenience of travelling to the single Jewish Temple.
You still kept the morally good single God rather than those embarrassingly degenerate polytheistic ones, and you put your money together and helped out your buddies in the group, which was a big practical help in an uncertain world. Also by combining a few cryptic passages in the old masters recorded speeches with some of the wilder philosophical speculations, he discovered that if you were good and stuck to the new rules, Jehovah would resurrect you after death and grant you eternal life.
It was a good package. This new cult proved popular and spread, not explosively but steadily, expanding converts by a few percent every year.
Remarkably this growth continued for centuries: normally people got tired of new cults and drifted away to others. Even more remarkably, it kept growing despite persecution; first by polytheists disturbed by dark rumours and the refusal of the cultists to sacrifice to their gods; then officially by the Roman state, disturbed by the thought of a rival authority to the Imperial cults.
Things really took off when a first would-be, then actual Emperor, called Constantine started favouring the new cult, probably even converting to it himself at some point. He recognized that their hierarchical structure of priests and bishops could be powerfully useful if they could actually agree on what they believed in.
There was a great diversity of beliefs at the time. Paul had shrewdly made the main focus of the religion the persuasive but safely out-of-the-way preacher Jesus the Anointed (Christ), not himself. But some of the Christians thought Jesus to be divine, some human, some followed more of the older Jewish laws, some thought there were two Gods one of the old and one of the new scriptures.
It was all a bit of a mess, so Constantine brought as many bishops as he could together at the Council of Nicea, dangling before them the vast and juicy carrot of acceptance as a state religion, and subtly thwacking the stick of more state persecution into the palm his hand. Eventually they thrashed out a compromise. Jesus was, um, wholly human and wholly divine.
Not everyone agreed to the new settlement of course, but a few decades of official persecution, now aided by their more cooperative Christian brothers, meant that those-who-believed-correctly (the "orthodox") kept a firm supremacy over those-who-chose-what-to-believe (the "heretics"). Half a century later emperor Theodosius finished the job of making Christianity the official state religion. By now the bishop of the Rome was capo di tutti capi.
So now the most persuasive religious deal in town was packaged up with the power of the Imperial Roman state. Popularity of the religion rocketed. When a few Emperors later one called Julian the Apostate tried to roll back the changes, the new religion was too strong, resisted, and made even more gains after his death. Christianity was now a big deal, and would remain so even as the power of the Western Roman Empire waned.
Islam and the Clashing Civilization
Monotheism was kicking polytheist arse all over Europe and around the Med. In 610 a shepherd turned merchant living in a trade centre in Arabia announced that he'd received a message from the One God that he was to create a new monotheist religion, a lot like Christianity plus a few Jewishish purity laws, and he was to be the boss.
Unlike the initial slow burn of Christianity, this religion spread rapidly from the start. The theoretical universality of the religion united the formerly fragmented Arabic tribes, and soon other ethnic groups as well, though they were not quite as equal in practice. The Arabic tribes had a strikingly new and weapon: camel cavalry. States that had thought themselves protected by desert wastes suddenly found themselves attacked out of nowhere. The remaining fragments of the Western Roman Empire, already weakened by plague, depopulation and political disorder, were rapidly overcome. But it wasn't just ex-Roman weakness: the Persian empire which had been duking it out toe-to-toe with Rome for centuries, fell too. Europe though, without much desert and where a camel was no more use than a particularly bad-tempered and ugly horse, didn't allow them such an advantage: they made few gains beyond Spain and Portugal.
Fortunately for Christianity, human nature had reasserted itself and the Islamic world split into factions. The Shiite and Sunni sects were rival descendents of the original prophet and wasted no time in starting to fight each other. This gave rise to rival dynasties, and there were ethnic tensions too.
The Eastern Roman Empire, centred in Byzantium, had always regarded itself as the true Roman Empire, and weren't too bothered by the decline of their savage cousins in the Western Roman Empire, who couldn't even speak Greek. They quickly learned that the Seljuk Turks, though theoretically fully committed to the universal brotherhood of the Muslim faithful, the ummah, were nevertheless perfectly happy to be paid by Christians to fight their stuck-up toffee-nosed Arab coreligionists. What with them, some random Viking warriors, a decent navy, some great big fuck-off fortified walls on their strategically nifty peninsula, and most importantly a lucrative control of trade routes to pay for all this, Byzantium seemed pretty secure against the rival Muslim kingdoms nearby. But that wasn't where the next problem was coming from.
The Great Schism and the Crusades
The Universal Christian Church lasted about up to the first major power struggle. At that point the Catholic Church in the Western Roman Empire split from the Orthodox Church in the Eastern Roman Empire in what was called the Great Schism, based on tedious differences of ego, politics and theology. However there wasn't much to fight about, so the different Churches mostly just eyed each other with mistrust.
Throughout the middle ages Christians who could afford it had made pilgrimages to Jerusalem in the Holy Land. It was always hard to differentiate a medieval minor noble from a psychotically violent thug, so these pilgrimages tend to take the form of a few hundred heavily armed nutjobs who might win or lose against the equally heavily armed bandits along the way. Travel was dangerous, and so were travellers. As populations rose in the late middle ages, the younger, but still psychotically violent, members of the noble orders got pretty restless, and the peasants too got hungry for land. Eventually the idea arose: why don't we all go on pilgrimage together and take over the place? It seemed like a pretty good idea all around, so Pope Urban II promulgated the First Crusade.
It was a long, boring, journey, though conveniently there were Jews along the way so you could start the killing and plundering early. In the Fourth Crusade though, the Western Crusaders found themselves temptingly close to Byzantium. It was still a long way to the Holy Land, the Orthodox weren't real Christians, and there a shitload of great stuff to plunder. So, they sacked the city and took it over. Eventually a fragile independent Byzantium reemerged, but by then the Westerners had all the trade routes, and their industrial base had disappeared as all the craftsmen fled. Byzantium was a hollow shell, and in 1453 Mehmet II cracked it open.
The Crusaders had established some coastal kingdoms which lasted a couple of centuries. It helped that a lot of the peasantry were still Christian and didn't particularly object to the new rulers on religious grounds. The new rulers could make money farming, and more money by acting as mercenaries, helping muslims fight muslims. This has mostly been possible because of that infighting though.
Eventually Saladin, a Kurdish muslim, managed to unite the other muslims enough to fight back against the Crusader kingdoms. Realising that the pilgrimage factor was a powerful motivator, he decided to make Jerusalem a Muslim holy city: up till then they'd never really been bothered about it. Despite resistance from the crusaders and fellow muslims hiring assassins to jump out of trees at him, he took back most of the territory.
A more successful crusade was the reconquista of Spain and Portugal by a series of Christian rulers ending with Ferdinand and Isabella. To reinforce their power over the mixed somewhat resentful population they created a Spanish Inquisition which rooted out real or imaginary Jews, Muslims and Heretics who might be sources of opposition.
Some have tried to call this period a Clash of Civilizations between Christianity and Islam. While there was plenty of clashing going on, the problem with this thesis is that most of the time people seem to have been perfectly happy to clash with fellow members of the same civilization, and ally with their theoretical enemy civilization if the money was right.
Catholicism to the Reformation
Christianity rapidly absorbed the moral, ethical and didactic aspects of classical philosophy. In particular the school of neo-Platonism with its dualistic ideas of a corrupt, unclean earthly realm and pure and spiritual heavenly realm was a major influence. Merging the two required a bit of tortuous shoe-horning of course. Judaism had carefully partitioned the earthly world into clean and unclean, but it had never occurred to them that there was anything wrong with the whole shebang. But the Song of Solomon was reinterpreted from earthly eroticism to a metaphor for God's horniness for his creation; the view of Resurrection was quietly changed from bodies levered up from graves to a Platonic immaterial realm, and the resulting theological kludge held together indefinitely for most of the eventual Christian splinter groups.
The religious texts had always been interpreted metaphorically of course, from the earliest Rabbinical texts to the earliest Christian theology. St Augustine around 400 AD was the prime mover in early theology. He pointed out that the bible could of course not be interpreted literally. For instance, the account of creation in Genesis could not even self-consistently refer to six literal days, since the sun hadn't been created till halfway through.
Some degree of philosophy remained, but it was firmly demoted to the position of the handmaiden of theology. Ethics, morals and exhortations to virtue would be in the religious domain now: philosophy could handle metaphysics, the natural world, language and the other boring stuff.
As the power of the Western Roman Empire faded, in central Italy the Catholic Church found itself accepting some temporal power. At first this was a reluctant necessity: Hadrian I needed some sort of civic infrastructure in Rome if only to keep the Church fed and watered: later popes were downright enthusiastic about it.
This tendency reached its heyday in the Pornocracy, or the Rule of Whores, when in the 900s a series of blatantly corrupt popes bribed or assassinated their way into the job, installing mistresses, lovers, and of course prostitutes into the Vatican. While this got some negative notice, on the international scene they were pretty effective rulers: negotiating peace settlements and expanding the church to Poland and Hungary.
Though the Pornocracy was long over, Church corruption was still an annoyance to some people. By the late middle ages, non-corruption was also a problem. Populations had risen, there were better communications, and local rulers were able to widen and deepen their power as the New Monarchies like France and Spain emerged. So naturally a rival source of power and wealth in the form of the Church was most unwelcome. There were even questions as to whether secular rulers could keep appointing church officials, the Investiture Controversy. Later in central Europe, some princes found a potential solution to their problem in the form of a discombobulated priest called Martin Luther who was kicking up a fuss about corruption and dodgy theology. The church had a nifty little sideline in selling indulgences, where for a reasonable fee they'd give you guaranteed time off purgatory. This annoyed Luther no end, especially since the pope was just frittering this money away on trendy so-called art by people like Raphael, Michelangelo and rebuilding Saint Peter's Basilica. Worse, as he pondered on this he began to doubt the whole notion as to whether the church could save people by "works", by which he meant the rituals and dispensations it controlled. So, the princes started to favour this Reformation, under a deal by which they would also conveniently control the church establishment in their lands. What could go wrong?
Unfortunately, it turned out that the common people really cared about this stuff. If you picked the wrong side in Catholic versus Protestant, you might be denied eternal salvation after all. What's more, the Protestant side started to fragment into its own factions. All of this led to the Thirty Years War, possibly the bloodiest and most brutal conflict ever to ravage Europe, which also led to conflicts all around the regions and tensions that would persist for centuries.
If the original Catholic Church had faded away things might not have been so bad, but they launched their own Counter-Reformation, cleaning up their act, promoting their lavish art in contrast to the dourness the protestants had adopted, and ruthlessly purging dissidents in their own ranks. It was at this point that oddball natural philosophers Galileo and Bruno fell foul of the Catholic church: normally favourable towards researchers, this was the wrong time to start questioning authority.
After the Catholics and the Protestants had fought themselves to an exhausted standstill, the churches had lost a lot of their money and secular power.
In the Nineteenth century in the USA, a curious new movement emerged, pejoratively called fundamentalism. Ultra-protestants had always been strong there: in the tumultuous period after the reformation many had seized the chance to escape oppression and persecution in their homeland and go oppress and persecute much worse-armed people across the Atlantic instead.
The fundamentalists believed in the five fundamentals, which amounted to protestantism on steroids. Protestants believed everyone, not just priestly experts, could interpret the bible: fundamentalists decided that the bible was self-interpreting and you didn't even need to know anything. They often favoured an ideal called dispensationalism: that on a predestined day a thing called the Rapture would happen and all the faithful would just get lifted up into heaven. Unlike all christians since Augustine at the latest, they thought the bible was entirely true on a literal, not just metaphorical level, even down to the mysteriously sunless days of creation.
I started writing this to try to correct some common misapprehensions that I keep seeing in religion arguments on Metafilter and elsewhere. Particular ideas are: the religions are homogenous blocs; biblical literalism and fundamentalism is universal and long established; the basket we label "religion" has always contained the same elements.
But essentially you can boil it down to this: religion is more complicated than you might think.
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