Do the large ammounts of bureaucracy, cult of personality around the emperor (who also held massive ammounts of power if compared to the kings of the Medieval age), social stratification, all-powerful armed forces, and imperialistic, xenophobic ideology not ring a bell?
It's no Airstrip One, sure, but the late empire did possess a lot of mechanisms for keeping the masses in line.
It doesn't ring any bell.
The strong power on political and military matters hold by the Roman Emperor wasn't totalitarian. There were nothing totalitarian in the Roman Empire. This country during the Principate didn't control their subjects, and local goverment was leave alone in hands of local elites.
I don't know about you, but if i were a non-Christian at that period, i'd find that more intimidating and bothersome than appealing.
Really, aside from the vague promises of salvation and the charity, i don't see Christianity really being that unique if compared to the other cults that were emerging at the same time.
Also the Christianity offered a strong "personal" conection to God, instead of sacrificies and rituals of pagan public religion. It was also a reason why various mysteries were popular at that time, or gnostic sects. Christianity also was apealing for lower classes, women, slaves, poor people. They weren't treaten by early Christians as worse element, as despicables. The unique characteristic of Christianity was its organization, since middle 2nd century Christian communities were lead by bishops with strong power, they were only onews who ruled in religious matters. Doctrine was also coherent, relativly simple, not very demanding, so it was actractive. There need to be something which caused this strong growth.
This... is not correct. You are assuming that Christianity was a massive portion of the Empire. At Paul's death it's a few people. Hardly a significant part of the Empire. You can't just handwave away the other competing religions away and assume that Christianity gets as absurdly lucky as it was OTL.
Your attack upon other faiths of the time as essentially not being real faiths is absurd. They had vast followings, particularly in the east of the Empire. They were quite well organized, and it's even said by the historian Dio Cassius that Mithras was mentioned in a state visit by Tiridates I of Armenia to Nero around this time. These "novelties" were certainly popular enough to gain following and notice even from monarchs. On the other hand, Christianity would be confused with Judaism for still some time to come past this time in history.
I know that during Paul's times it was the small group, but you missed my point. I wrote that Paul and his friends did the POD, which allowed further christianian expansion outside being a small messianic jewish sect. It is the key, the fundation of the future sucesses.
It isn't absurd, it is a statement of facts, that despite being described as rivals to Christianity, they weren't real rivals, but just as I described "novelties" with limited followings, and a lack of coherent, universally apealing doctrine. They never got even close to a seize of Christianity say in 3rd century. When Christianity was that big, that Decius' decreee about sacrificies was clearly against them (Jews were relased from this obligation). Those cults were just elements of the wast landscape of religiosity, contrary to Christianity or Judaism, which presented themselves as the one and only true religions. There is a difference between a "absurd luck" and a systemic causes of Christianization. "Absurdly lucky" might be Alexander, that he didn't die from any of his wounds, not Christianity, which was growing by its own internal reasons and atractivness for the population.
For Mithra, you are also wrong. The Roman Mithraism with its bull sacrificies weren't the same as a worshiping of Mithra and ancient indo-iranian god, which is known even in Vedas. The Romans adopted an iranian name, some aspects, but created a new thing. For the armenian king, there probably mean an old iranian Mithra.
The Romans stopped to confuse Christianity with Judaism relativly early, Pliny certainly didn't confuse them, although his knowledge of Christianity was limited.
The Sassanians managed to reform a decentralized and polytheistic religion into an organized state religion. Why not the Romans?
The Zoroastrianism of the Arsacid/Parthian era in fact has significant similarities to the pre-Christian Roman religious situation, with a mix of Greek and Iranic deities being worshiped side-by-side, regional variations in religion, and cult-deities predominating. It's not known whether Zoroastrianism had been unified in the first place in the previous Achaemenian era, but by the Arsacid/Parthian era it had certainly been greatly displaced by Hellenistic and other belief systems. The Sassanians managed to transform this into an official religion intertwined with the state bureaucracy, with a strong priesthood, redefining the holy centers, compiling new holy texts and even viewing the new religious orthodoxy with a degree of proto-nationalist fervor. Additionally, evidence suggests that the Avesta was first committed to a written form in the Sassanian era.
Given such historical developments in Zoroastrianism, why can't a pagan religion with some degree of institutions and literature codify and spread itself?
It is a diffrent situation. The codification/redaction of existing holy texts, and uniformisation of existing doctrine versus creating a totally new stuff, which was needed for Julian's religion.
The hellenization of the iranian people during the hellenistic and parthian periods was very limited. They even didn't adopt the hellenic script for their languages, (except far far away Bactria).
You are right in that we don't know much about the evolution of Zoroastrianism. But also it is important that this religion had already existed before the Sassanians, yet not in uninformed form, intervined with the State. Just like Christianity existed had existed before the formation of the State Church of the Roman Empire, with Emperors who organized councils and decided on doctrinal matters. The Avestan Language is much older than the Sassanians, so holy texts had to existed way before, but of course probably in many diffrent versions.
But Julian contrary to the situation of the Sassanians needed to create a new stuff. A new universal, organized, apealing, coherrent religion. An impossible task, in my opinnion. Becaue if he succeded with that the would have did a "pagan Christianity", I mean a religion with a diffrent names of gods, beliefs, but with a similar organization and probably also the end of this famed "polytheism tolerance". Because if he wanted for this to be a religion for the Empire, he needed to establish a dominane of this above others, a diffrent stuff than the religious indeference characteristic for the ancient polytheism, which only demanded some rituals, sacrificies, etc performed publicaly as important for the comunity (in a matter of a city-state, how it was with the roman religion, or hellenic).