#1- But leaders of a religion that preaches for people to repent before the End Times can remember what their religion is supposed to be about before they curry favour with an imperial overlord that happens to think they're useful. Some church leaders clearly didn't believe in what they preached. Christianity's success in the early Fourth Century CE had less to do with what the general populace thought of it then the contemporary powers-that-be.
#2 - So, people should be characterized generally as selfish and spiteful bastards who enjoy seeing others struggle and suffer while expecting an eternal reward for just grovelling in regret about their own mistakes or trespasses?
#3 - Well even if this was true of most of the human race, this argument in favour of Christianity's so-called mass appeal always fails to take into account the actual genuinely held beliefs of people from other religions. No matter how good it may sound on an individual basis, not everyone is willing to jump on board, whatever promises are made.
#4 - There were lots of people at the time who thought that Zeus/Jupiter was the mightiest being in the cosmos and were as unwilling to cross him just as many believing Christians today are worried about their god's displeasure. When a large demographic are raised from infancy in a belief, its difficult for them to shake off when they're adults. No religion is unique in that.
#5 - I really doubt that even the very first Christians were uniformly chaste. Some early Christians raised families. Marcion was the son of a leading Christian in Sinope and inherited a shipping business. Chasteness, while considered a virtue by some, wouldn't have been realistically observed by everyone.
#6 - Before the 300's CE, Christians didn't number more than 10% of the empire's population. Constantine's endorsement of the cult and ending the persecutions started by Diocletian and Galerius was a reversal of their fortunes. Their numbers seriously started to expand from that point onwards because they were generally favoured by the state. This increased patronage by the emperors, who had all been raised Christian starting with Constantine's sons, helped the Church grow as an organization and was even granted a place in civil administration, and the leading bishops acting as politicians and imperial advisors. This increase in special treatment encouraged elements of the aristocracy to convert.
#7 - By the reign of Theodosius I, traditional ethnic religions were banned and their practices were to be considered treasonous. The common people were then legally obliged to convert. The new decrees were better applied in the cities than in the countryside, so this is what led to "Pagans" getting that label. Hellenic Polytheism continued to linger right until the Ninth Century in when the last known practices of the religion Lacedaemonia when St Nikon the Metanoeite reputedly ended them.
#8 - Christianity had a full-time clergy that could double as a bureaucracy, which is why it appealed to many post-Roman kings. The Romanized-Hellenized religious cults had priests with only part-time religious duties and didn't possess a tight corporate unity. The Church is the reason why Christianity was successful. Its so-called "messages" less so.
(Numbers added by myself at the head of each section so I can adress them in order)
#1 That is a central criticism of any religion pagan or otherwise. Of course some members of the early Chruch were power hungry and greedy, so were many of the pagan priests of old, they simply had to expansive a pantheon to properly integrate thenselves into the political system. The Christian Church OTOH had a central system, One God to follow and a fairly standard set of beliefs. Just try making one of the pantheons like the Catholic Church. It would be pretty damn difficult, especially without the exclusive nature of Christianity.
#2 Thats one of many schools of thought regarding human nature.
#3 Well despite those genuinely well held beleifs of other cultures Christianity did spread pretty damn quick and those other genuine beliefs died pretty quick considering their previous entrenchment. That they didn't stick nor even slow the spread of Christianity in any meaningful way is pretty relevant.
#4 hand in hand with #3
#5 I doubt that too, but the practice and belief was there...
#6 By 300AD they were close to 10% and their numbers were gaining, Constantine's edicts had little effect on their growth. In fact they spread quicker after the Western Empires demise. It was the so called 'barbarians' who spread it rather than Rome. Its also important to remember that much of Europe wa still pagan for many centuries after too.
#7 That probably allowed them to last longer than they did actually. Resistance to the ban helped increase growth throughout many parts of the Empire until its eventual death.
#8 Not denying their power as a central Church helped them spread but I think you underestimate just how much people had a habit of caring about their immortal souls back then. The kings who saw a chance to save themselves and earn paradise in the next life would jump at that as a pretty good deal, the fact that some monks might be willing to be scribes or to train them was simply a bonus. I don't think the writing prowess was as impressive to the masses, nor does that in any way shape or form even come close to a good reason for explaining its spread.