I am by no means an expert on this in any way. This is just my own guess off the top of my head, based on what little I've read on this subject.

The biggest factor I can think of is that the Romans were oppressing the Jews and the Jews wouldn't have it. But the Jews were so outmatched militarily that any revolts they had were easily squashed and it seemed pretty damned hopeless for the Jews. Their religion was in the process of potentially going extinct. The old ways weren't 'working' anymore, and so a whole bunch of Jews became disillusioned and started looking for 'something new'.

Possibly: The idea started to float around that 'the end is coming', and 'the messiah is coming', or something along those lines, and so the field was ripe for the kind of theology that ended up becoming 'Christianity'. There were apparently very very many early 'Christianities' and they were all quite different and not unified. It is possible that within this cultural/religious context, the idea of a kind of spiritual saviour could have arisen and spread rapidly by word of mouth, along the lines of how various woo woo fads spread around by word of mouth these days (think of 'the Secret', quantum woo woo bullshit, and that kind of thing).

Once a lot of superstitious and fearful Jewish people believed in the possibility of such a 'saviour', the stories and superstitions could have grown rapidly around that simple seed idea. Eventually, you get a bunch of self-proclaimed prophets/messiahs/saviours/etc. all making up their own bullshit claims connected to this idea, and they start to gather followers.

Then there are the inevitable theological disputes, and a kind of cultural evolution eliminates the 'weak' religions/cults and leaves behind the 'stronger' ones, which by their nature become more and more dogmatic and rigid. This is the context that Paul wrote within, and his writings are examples of.

And eventually, 300 or so years later, you end up with Constantine making one particular branch of it the official state religion. Paul's theology just happened to be at the core of that particular branch.

I defer to the actual historians, anthropologists, and others who have some real expertise in this area to provide the details and corrections to my cartoon sketch. I'm just saying it's not so implausible to me.

                       Julius Caeser wrote about devout Judaens who lived in his mothers apartment building, he learned Hebrew from them. In order to avoid trubble with the Roman religious authorities, they would take a short walk to Vatican Hill and make offerings in the Main Temple of Mithras. This was commen among all non-Roman ethnic groups living in Rome.  Caeser wasn't writing about all Jews, just the few dozen familys he knew in his own neighborhood. This is around 90 BCE, so the Jews and Mithra combo is going on before that.  When you combine Yawah with Mithras you get Christianity.

                        Who is Mithras?  The son of god according to the Zoastrians, the light of light, the prince of peace; surprising since it was considered the soldiers religion. He was born on Dec 25,  of a virgin.  He had 12 apostles, when he died he took away the sins of the world but arose 3 days later just in time for the spring furtility festival of eostrogean [Easter],  he even had a big bash 'last supper' with his apostles before dieing. Does any of this sound familier?

                        Jesus Christ [or Joshua Massiah] is NOT a name!!  It is a title for the long awaited savior of the Hebrew peoples; that is what the title means 'gods' savior, annointed' .   Between 30 BCE and 30CE there may have been 100 or more wondering preachers between Gallilee and Jerusalem. Each had a following and no doubt the more ardent of followers considerd their man the Massiah Joshua [the Christ Jesus].    Some of the names survive, Apolonia of Tiere, Baptiser Iaonnes, etc.  Evidence for the multiple sources for Jesus is all over the NT, just ignored. Matt 5 [the beatitudes] as JC sounding like Mr.Roger's neighborhood; whereas Matt:10 as  JC sounding like  Mr.T [..I come with a sword..]. That is at least 2 very different preachers.

                      Jesus the Nazarene,  refers to Jewish cult called Nazarenes, not a home town [Nazarite]. Nazarines were strict astetics; No drinking, No dancing, No fun at all, just  adherence to Yawah and his laws.   Compare that to the JC who was drinking wine at the drop of a parable when ever he could. That version of JC was no  Nazarine!  That does create 2 more sources for JC.

                      Wine drinking is interesting, The Hebrew tribes were introduced to wine by the Greeks in the forth century BCE, they took to it real fast, the Romans later introduced the worship of the wine god Bacchus. Bacchus was invoked during weddings and other big partys. When the wine ran low partiers prayed to Bacchus to 'turn our water into  wine', also any good Roman host would see his parting guests to the door for one more sip of wine with the words "drink this in rememberence of good times" or "drink this in membrence of me." Does this sound familier?

                       So now we have Bacchus & Yawah getting together [cir]330 BCE,   Mithras & Yawah getting together before 90 BCE , itinerate preachers and their PR squads tacking their 'mans' teachings on top of the Hebrew bible, Some of that PR work was written down and some was carried around by word of mouth.  Then Saul of Tarsus, changed his name to Paul and took it on the road. Paul was actually in the process erasing these unorthodox Jewish churches when he had his epileptic siezure on the road to Damascus.  He woke up a brain damaged Christian.

                     Unlike other religions Paul insisted that Christians take their new religion out and recruit has many new adherents as possible, Evangilize the world. This new religion with it's message of salvation and familier Roman traditions played well with the lower classes and the slaves of Rome; i.e. 90% of the Roman Empires population. It moved fast through that desperate population but it still took 300 years before the Mithras temple on Vatican Hill headed by a pope to become the leading Jesus temple headed by a pope.